The molecular modeling of the transition metal complexes (TMC), reproducing characteristic features of their stereochemistry and electronic structure, is one of the important goals in the modern computational chemistry. The need for such modeling arises while studying and developing various processes involving TMCs.
The tools generally available for performing a required modeling are inconsistent with the problem. Quantum chemistry (QC) in general seems to be indispensable for performing the required modeling since the number of bonds (or coordinated ligands) may become variable. Also the presence of the open dshell of the metal ion requires quantum mechanical modeling tools since a number of the electronic (spin) states of the complex closely lying on the energy scale arises, and thus the PESs belonging to different electronic terms closely approach each other, or even cross, which manifests itself in experimentally observed spin transitions [] or JahnTeller distortions []. However, employing ab initio QC methods for the purpose of TMC modeling faces serious problems since the molecules under consideration are generally rather large whereas the computational costs of the methods to be used in order to reach at least an acceptable result scale up to N^{7} with the system size (N is the number of AOs in the molecule) due to the importance of electron correlation.
When a similar problem, the necessity of modeling of large molecular systems like polypeptides or simply polyatomic organic molecules, is faced by örganic" QC, semiempirical methods are usually applied. With clearly identifiable exceptions like the long polyene chains or systems with breaking sbonds these methods in general perform quite reasonably giving relative heats of formation within series of related molecules and corresponding molecular geometries with "chemical" precision. The progressive route of improvements in the semiempirical methods as applied to örganic" molecules consisted in (i) increasing the number of the Coulomb interaction integrals taken into account, (ii) the sophistication of the corecore repulsion terms added to the semiempirical electronic energy. However, the numerous readjustments of the parametrization of the traditional semiempirical protocols were not too much profitable for their usage for modeling of TMCs which previously faced and still faces severe problems. These problems are widely known in the literature as reviewed in Refs. [,]. Briefly they can be formulated as follows:
In a more general aspect one can state based on the analysis even of the recent attempts of semiempirical treatment of TMCs [,,,] that the errors can be generally characterized as unexpected and nonsystematic deviations in the calculated ground state spins and symmetries from the corresponding experimental data. The long persistence of these problems (in fact for several decades) indicates their fundamental rather than technical character. Something is wrong with the very idea to parameterize the traditional HFRbased semiempirical description for the electronic structure of TMC.
The paper is organized as follows. In the next Section a formal point of view on the problem of constructing a semiempirical description of TMCs is presented and applied to analysis of the corresponding difficulties. Further Sections are devoted to description of a semiempirical procedure of effective Hamiltonian of crystal field (EHCF) satisfying the validity conditions to be used for TMCs' modeling and of its application to analysis of spinactive complexes of iron (II). Finally discussion and conclusions are given.
The relevant formal treatment starts from the notion that all the
quantities related to the electronic structure of molecules can be
calculated with use of only one and twoelectron density matrices
for the relevant electronic state of the system under study
[]. Taking the energy for the sake of definiteness we get:
 (0) 
In practice the density matrices are constructed as partial integrals of the
corresponding trial wave functions Y_{C[(Q)\vec]GS}(xw  x_{1},x_{2},¼,x_{N}) for the given composition, nuclear configuration, and
the specified total spin and spatial symmetry:
 (0) 
 (0) 
The expressions Eqs. (1,2,3) are
completely general. In order to adjust these technique for using in
relation to the TMCs' modeling we have to consider a circle of
concepts known as electron correlation []. Indeed,
the statement that the motion of electrons is correlated can be
given an exact meaning only with use of the twoelectron density
matrix Eq. (2,3). Generally, it looks like
(with the normalization of paper [])
 (0) 
The first term in this expression expands as
 (0) 
All the technical tricks invented to go beyond the HFR calculation scheme in terms of different forms of the trial wave function or various perturbative procedures reduce in fact to attempts to estimate somehow the second term c in Eq. (4) which is the cumulant of the twoparticle density matrix [] responsible for deviation of electrons' behavior from the model of independent fermions. It comprises the qualitative features of electron correlations.
The above considerations may seem to be too much theoretical and to lay too far from the practical needs of modeling electronic structure of TMC's. It is not true, however. It is easy to understand that the nature of unsuccess of the long lasting attempts to construct an acceptable semiempirical parametrization for the transition metal compounds within the framework of the HFR MO LCAO paradigm lays precisely in the inadequate treatment of the cumulant of the twoelectron density matrix.
The procedure of developing a semiempirical parametrization can be
formalized in terms of Eq. (1). From this point of
view a set of experimental energies E(C[(Q)\vec]GS)
corresponding to different chemical compositions C, molecular
geometries [(Q)\vec], and electronic states with specific values of
S and G is given. Developing a parametrization means
to find a certain (sub)set of parameters w which minimizes
the norm of the deviation vector d[(E)\vec]_{w} with the
components E(C[(Q)\vec]GS)E(C[(Q)\vec]GS  w) numbered by the tuples C[(Q)\vec]GS:
 (0) 
 (0) 
 (0) 
 (0) 
with the upper sign corresponding to S = 0 and the lower one to S = 1, irrespective to the values of the subscripts C[(Q)\vec] and parameters xw. Opening the brackets in the above expression gives rather long formula which contains the part depending on the total spin i.e. which is different for the singlet and triplet states of the same spatial symmetry B. The physical consequences of this difference are well known: it is what immediately leads to the first Hunds rule stating that in an atom the term of a higher spin (under other equal conditions) has lower energy. The energy difference between these terms is nothing but the exchange integral. By this we clearly see that the situation we face in TMCs is intimately related to the (grammatically) correct treatment of the cumulant of the twoelectron density matrix. Two states of say dshell differing by the total spin only must have different energy whereas the HFR theory does not provide any quantity to which this difference can be anyhow ascribed. Notice, that the problem is not in the type of the Coulomb exchange integrals whether appearing or not in the parametrization scheme, but in its density cumulant counterpart the integral must be multiplied by. Even in the case when the HartreeFock part of the twoelectron density matrix provides a multiplier to be combined with that or another type of exchange integrals which are responsible for the energy difference between the states of the different total spin, in the absence of the necessary component of the twoelectron cumulant this difference remains zero anyway. In more complex situation than that of two electrons occupying each its orbital one can expect much more sophisticated interconnections between the total spin and twoelectron densities than those demonstrated above. In any case they are concentrated in the cumulant. This explains to some extent the failure of almost 40 years of attempts to squeeze the TMCs into the semiempirical HFR theory by extending the variety of the twoelectron integrals included in the parametrization.
We do not intend to further elaborate on characteristics of the widespread semiempirical methods. It is enough to say, that all of them which are restricted to the HFR approximation suffer from the shortcoming described above and, hence, one has not to have too much hope to reach a consistent description of TMCs within their framework. The recent semiempirical attempt to develop a parameterization for the transition metal compounds is the PM3 (tm) method [,]. It is intensively applied to calculations of various TMCs. The calculations carried out [,] show that the method is not capable to reproduce even very simple characteristics in a series of TMCs having similar structure, though other authors [,] state that in some cases reasonable estimates of geometrical characteristics may be received, nevertheless. This situation can be understood by thorough analysis of the sets of complexes used by different groups of authors. In papers [,] authors study the uniform set of about 30 complexes of Ni^{2+} with the ligands bound by the nitrogen donor atoms. The analysis of this series performed there clearly shows that PM3 (tm) fails for Ni^{2+} for the now understandable reason. In the papers [,] the authors by contrast try to explore a comparable number of complexes but much more dispersed over the range of molecule classes which includes both the first and second row transition metal complexes, high and lowspin ones, those having ïonic" and "covalent" bonds etc. In this test set the problematic classes of compounds are presented by a couple of examples each and look out to be completely isolated exceptions. This can serve as an example of how trying to test the method on a wide and apparently "random" selection of objects may lead to a smeared picture due to absence of clear criteria designed to introduce an adequate classification within the chosen set. On the other hand we have to mention that the semiempirical method ZINDO/1 [] which allows for some true correlation by taking into account the configuration interaction may be considered as a setting prospective for further parametrization.
The grim prospects to obtain the semiempirical HFRbased picture of electronic structure of TMCs which follow from the above analysis are in a sharp contrast with the situation with the empirical understanding of detailed features of the latter. The description of TMCs used to interpret and analyze their UVVIS spectra, magnetic and partially structural properties is based on the crystal field theory (CFT) [,] allowing qualitatively correct description of these characteristics of TMCs.
The CFT was introduced in the classical work by Bethe [] devoted to the description of splittings of atomic terms in crystal fields of various symmetry. The qualitative pattern of this splitting is established by considering the change of symmetry properties of atomic wave functions while lowering the spatial symmetry from the spherical one (in case of an atom) down to the symmetry of a point group of the crystal environment. To calculate the energies of the ddexcitations in this model, it is necessary to diagonalize the matrix of the Hamiltonian constructed in the basis of n_{d}electronic wave functions (n_{d} is the number of delectrons). Matrix elements of the Hamiltonian are expressed through the parameters of splitting (10Dq  in case of an octahedral field) and those of the Coulomb repulsion of delectrons, that are the SlaterCondon parameters F^{k}(dd), k = 0,2,4, or the Racah parameters A, B, C related to the former. In a simplest version of the CFT these quantities are taken as empirical parameters and determined from comparison of the excitation energies, calculated within this ionic model, to the experimental ones. Such an approach allows to describe with high accuracy the spectra of lower excitations of the impurity ions in crystals and of the TMCs, and in many cases to assign successfully the absorption bands observed experimentally.
Although the predictive force of the described approach is lost due to presence of empirical parameters in the theory, which makes it dependent on completeness of experimental data, the CFT correctly reproduces the basic qualitative features of the electronic structure of transition metal ions in crystals and TMCs well known to chemists. These features are the presence of specific group of electrons in the dshell and symmetry of the external field which both determine the form of the spectrum of lower excitations. Analysis of the basic assumptions and constructs of the CFT shows its striking difference from those of the semiempirical HFRbased methods. Indeed, the CFT restricts itself with the electrons in the dshell only, whereas the HFRbased semiempirical methods extend their description to all valence electrons. On the other hand the CFT uses possibly the most precise form of the electronic wave function: the full configuration interaction (FCI) one in the space of the considered oneelectron states, which allows to reproduce all the components of the cumulant on the twoelectron density matrix "block" which relates to the corresponding subset of the oneelectron states. Incidentally, the problem of simultaneous description of several manyelectron terms of similar occupancy of the oneelectron states but of different total spin arises in the context of molecular modeling namely when it goes about TMCs and more precisely about the states of their open dshells. The latter preserve largely their characteristics inherited from the free atoms or ions and thus the corresponding system of multiplets. The latter to be reproduced requires as it is shown above a proper description of electron correlations or equivalently correct form of the twoelectron density matrix cumulant. The HFR wave function on the other hand neglects all the nontrivial parts of electron correlation. Our belief is that the above difference reflects the distinction between the details of electronic structures of the compounds described by them. The HFR form of the wave function springs from the Hückel method. In early years of development of quantum chemistry this type of the wave function had been applied to örganic" molecules like benzene and other aromatic compounds whereas the constructs specific for the CFT were used to describe ïnorganic" molecules and materials reproducing by this the separation of chemistry itself into organic and inorganic and thus taking into account specificity of compounds related to these two classical subtopics of chemistry. The essential differences in the corresponding electronic structures are reflected in the form of the trial wave function accepted as zero approximations. Further development, dominated largely by numerical methods stemmed from the Hückel form of the wave function rather than qualitative reasoning, faces problems when addressing open dshells. The general theorems (the Löwdin theorem, for example) are not of much help here since they are existence theorems which only state the possibility of obtaining the exact manyelectron wave function as an expansion over Slater determinants composed of orthogonal oneelectron states, for example, coming from the HFRMOLCAO procedure, but says nothing about how long an expansion giving an acceptable accuracy is going to be.
The CFT by contrast gets directly to business when it goes to TMCs' description. All key features of the electronic structure of TMC are fixed in the structure of this theory and the only problem is a consistent and independent estimation (calculation) of parameters of the crystal field induced by the metal ion's environment. All the development of CFT was concentrated on this problem. It is the basic difficulty, that has no solution in the framework of the CFT itself. The reason of this failure is transparent enough and consists in oversimplified description of the transition metal ion environment with the purely ionic model. It neglects all electrons outside the dshell and takes into account only the symmetry of the external field and electronelectron interaction inside the dshell. This deficiency can be lifted by considering the CFT as a special case of the effective Hamiltonian theory for one group of electrons being part of an Nelectron system where other groups of electrons also present. This allows a sequential deduction of the effective Hamiltonian for the dshell as performed in Ref. []. By this the matrix elements of the said Hamiltonian are expressed through characteristics of the electronic structure of the metal ion's environment. A brief description of the corresponding moves is given in the next Section.
The deduction [] of the EHCF is based on representation of the wave function of TMC in a form of the antisymmetrized product of the group function for delectrons and of that for other (valence) electrons of a complex which separates the electronic variables. The detailed analysis of the formal elements of the electronic structure theory together with that of the electronic structure of TMCs, distinguishing these from molecules not containing transition metal atoms reveals the basic features which must be taken into account when developing an adequate semiempirical description of their electronic structure:
These features on one hand constitute the physical reasons why the HFR based approaches do not apply for the description of electronic spectra of TMCs, and on the other hand prove the necessity of the localized description of delectrons of transition metal atom in TMC with explicit account for effects of electron correlations inside the dshell. This can be done if one explicitly takes into account correlations of electrons in the dshell of the transition metal atom (mentioned further as the dshell). In a zero approximation for the TMCs' wave function one may use the function formalizing the CFT ionic model, i.e. one with a fixed number of electrons in the dshell. The interactions responsible for electron transfer between the dshell and the ligands can be considered as perturbations. Following the standards of semiempirical theory we restrict the AO basis for all atoms of TMC by the valence ones. For the metal ion the vacant 4sand 4porbitals are included. All the AOs are separated into two subsets from which one (the dsystem) contains 3dorbitals of the transition metal atom, and another (the "ligand subsystem", or the lsystem) contains 4sand 4porbitals of the transition metal atom and the valence AOs of all the ligand atoms. Furthermore, we consider only such complexes, where excitation energies in the lsystem are by far larger than the excitation energies in the dshell of the metal atom. This singles out a subset of the Werner TMCs which can be alternatively characterized as ones with the close electronic shells ligands, such as F^{}, Cl^{}, Br^{}, I^{}, saturated organic molecules with donor atoms. This comprises the set of objects we are going to cover in this theory.
Formally the theory evolves as following. The electronic wave
function for the nth state of the complex is written as the
antisymmetrized product of wave functions of the electron groups
introduced above:
 (0) 
 (0) 
 (0) 
 (0) 
 (0) 
 (0) 
 (0) 
 (0) 
 (0) 
In the context of the EHCF construct described in the previous Section the problem of semiempirical modeling of TMCs' electronic structure is seen in a somewhat different perspective. The EHCF implicitly contains the crucial element of the theory: the block of the twoelectron density matrix cumulant which relates to the dshell. Instead of hardly justifiable attempts to extend a parametrization to the transition metals it is now possible to check in a systematic way the value of different parametrization schemes already developed in the örganic" context for the purpose of estimating the quantities necessary to calculate the crystal field according to prescriptions of the EHCF theory Eqs. ( 14)  (16). Solving the wave equation for the effective Hamiltonian for the dsystem H_{d}^{eff} with the matrix elements which are estimated with use of any örganic" semiempirical scheme with the CI wave function constructed in the basis of the dsystem, we obtain the complete description of the manyelectron states of the dshell of the metal ion in the complex. In such a formulation the EHCF method was parameterized for calculations of various complexes of metals of the first transition row, with mono and polyatomic ligands. In the works [,,,,] parameters for compounds with donor atoms N, C, O, F, Cl and doubly and triply charged ions V, Cr, Mn, Fe, Co, Ni are fitted. These parameters do not depend on details of chemical structure of the whole ligand, but are characteristic only for each pair metaldonor atom. The dependence of the excerted effective field on details of geometry and chemical composition of the ligands are believed to be reproduced in a frame of a standard HFRbased semiempirical procedure used to describe ``organic'' lsystem within the proposed hybrid approach. The further evaluations [,,] have shown applicability of the fitted system of parameters for calculations of the electronic structure and spectra of numerous complexes of divalent cations with use of the CNDO parametrization for the lsystem. In Refs. [,] the EHCF method is also extended for calculations of ligands by the INDO and MINDO/3 parameterizations. In all calculations the experimental multiplicity (spin) and spatial symmetry of the corresponding ground states were reproduced correctly. The summit of this approach was reached [] in the calculations on the complex cis[Fe(NCS)_{2}(bipy)_{2}]. The molecular geometry is known for both the high and lowspin isomers of the said compound. The calculations reproduce the respective ground state spins and the spectra of low lying ddexcitations in remarkable agreement with the experimental data.
Another semiempirical implementation of the EHCF method is based on the well known SINDO1 scheme developed by Jug and coworkers [,,]. The SINDO1 scheme has some specific features which seem to be very important in the light of the EHCF formulation. First of all, the explicit account of the nonorthogonality of the atomic orbitals by using a Löwdin orthogonalized basis set provides a validity of the strong orthogonality condition between the group functions F_{d}^{n} and F_{l} and justifies the derivation of the system of the effective Schrödingerlike equations for the electronic subsystems of a TMC. The second quite important point is that the SINDO1 method uses a theoretically justified and well parameterized expression for the semiempirical resonance integrals. This expression makes it possible to reproduce the features of the distance dependence of the resonance integrals in the relatively wide range of bond distances.
The details of the EHCF/SINDO1 implementation are described in details in []. Here we will briefly outline the specific features arising from using the orthogonalized atomic basis sets in the parameterization scheme.
The singledeterminant wavefunction F_{l} for the ligand subsystem is calculated with slightly modified SINDO1 method using the effective Fockian for the lsubsystem. First, the matrix elements of the core Hamiltonian of the lsystem are renormalized to reflect the interaction of the ligand electrons with the electron density in the dsystem. At the next stage, the effective core Hamiltonian matrix is transformed to a symmetrically orthogonalized atomic orbital (OAO) basis set. This transformation takes into account the dorbitals of the metal atom as well, so that the resulting OAO basis set in the lsystem and the transformed dorbitals of the metal are orthogonal to each other. The resulting effective core Hamiltonian matrix elements in the OAO basis for the lsystem include two types of the firstorder orthogonalization corrections originating from ligand atomic orbitals and metal dorbitals, respectively. The twocenter offdiagonal matrix elements of the core Hamiltonian in the OAO basis (resonance integrals) have additional empirical correction terms with adjustable pair parameters fit to reproduce the geometries, heats of formation, and ionization potentials for a representative set of transition metal compounds [].
Compared to the Eq. (13) the effective oneelectron
parameters for the dsystem U_{mn}^{eff}
in ENCF/SINDO1 scheme contain two additional contributions
 (0) 
 (0) 
 (0) 
The term W_{mn}^{orth} is a first order
correction originating from the Löwdin orthogonalization
of the d orbitals with respect to the ligand orbitals:
 (0) 
 (0) 
The EHCF/SINDO1 method has proved to be useful for calculations of spectra of lowenergy excitations in some iron(II) complexes and ionic crystals []. In all cases the method reproduces not only the experimentally observed spin and symmetry of the electronic ground state but also provides the excitation energies with a good accuracy. The calculation of the splittings of the dlevels of the metal complex with partitioning of the total splittings into various contributions according to Eq. (19) gives a unique possibility to analyse the details of the electronic structure of the complex in simple terms analogous to the Crystal Field Theory, so widely used by inorganic chemists for the interpretation of the electronic spectra of these compounds.
The quadrupole splitting measured in the Mössbauer spectra of iron compounds is due to the interaction of the quadrupole moment Q of the ^{57}Fe nucleus in its excited state and the electric field gradient (EFG) at the position of this nucleus. The EFG at r = R^{(Fe)} in the presence of the external charge density r(r) (electrons and other nuclei) is represented by the traceless tensor with the components
 (0) 
 (0) 
In a line with the EHCF representation of the total wavefunction
as an antisymmetrized product Eq. (10) of the group functions for
the dsystem and lsystem the total EFG tensor for the TMC in its
nth electronic state can be expressed as a sum of two contributions
 (0) 
 (0) 
The second contribution V_{ab}(n_{l}) is due to the electrons and nuclei
of the lsubsystem. It can be further partitioned into the contribution of
the valence 4pelectrons of the metal atom V_{ab}^{4p}
and the contribution of the effective charges on the ligand atoms V_{ab}^{L}
(the contribution of the valence 4s electrons vanishes due to the spherical symmetry)
 (0) 
 (0) 
 (0) 
The oneelectron matrix elements
ám v_{ab}n
ñ
and
á p v_{ab} q
ñ
have the following structure
 (0) 
 (0) 
Due to the presence of electrons in the inner shells of the iron atom
the contributions to the total EFG tensor have to be corrected
to reflect the shielding and antishielding effects of inner electrons.
The shielding effects for the valence 3d and 4p electrons and antishielding
effects for the charges on ligand atoms are described by the Sternheimer
factors 1R and 1g_{¥}, respectively []
(in this work we adopted the values R = 0.32 and g_{¥} = 9.1
used previously for Fe(II) complexes [,]).
With this taken into account the total EFG tensor is represented as
 (0) 
The total temperature dependent EFG tensor V_{ab}(T) can be
calculated by averaging all the components over all the excited electronic states
acording to Boltzmann statistics
 (0) 
Finally, the quadrupole splitting DE_{Q} in ^{57}Fe
Mössbauer spectra is given by the expression
 (0) 
 (0) 
In its simple version the EHCF/X method treats the electronic structure of
the ligands within a semiempirical approximation X. These methods
are not, however, designed to conduct the systematic studies of
potential energy surfaces (PESs) of TMCs. Further application of the
EHCF methodology would be to develop a method for the calculation of
PESs of TMCs. The CNDO or INDO parameterizations for the ligands are
probably of high enough accuracy when it goes about the charge
distribution in the ligands and the orbital energies at fixed
experimental geometries. However, these methods do not suit for
geometry optimizations (or more generally for searching PESs) of
TMCs. Applying the electron partitioning into groups []
allows to effectively formulate hybrid schemes of the QM/MM type.
In the original implementation [] EHCF method does
not allow to follow the PESs of TMCs.
Nevertheless, the EHCF method can be adapted for this application in
a framework of the hybrid scheme QM/MM. Indeed, according to
[] complete electronic energy of the wave function (14) in
its nth state is:
 (0) 
 (0) 
The general scheme of the energy evaluation Eq. (38) is based on the implementation of the EHCF method in which the wave function F_{l} for the lsystem is calculated within a semiempirical version of the HFR approach (for example, CNDO) and then used to construct H_{d}^{eff}. This scheme appears rather expensive to be used for searches of PES, since it requires the HFR calculations on the lsystem for each geometry of the complex. To clear this, we have developed the local version of the EHCF method which allows to calculate the crystal field much more economically.
Not going into too much details we can say that the local EHCF method developed for this purpose allows to calculate the covalent contribution to the effective crystal field through the characteristics of ligands' lone pairs. The local EHCF method was implemented and used for the analysis of the molecular geometries of complexes of iron (II) in works [,]. In our work [] we describe the effect of electrostatic field of the metal ion on the ligands within the electrostatic polarization model. The appropriate objects allowing to test the described approach are spin isomers of TMCs. Satisfactory precision of the estimates of geometry dependence of the effective crystal field in a series of complexes of iron (II) and cobalt (II) (both low and highspin ground states) is achieved []. We considered totally 26 sixcoordinated iron complexes with mono and polydentate ligands, containing both aliphatic and aromatic donor nitrogen atoms (mixed complexes with different types of donor nitrogen atoms and different spin isomers of one complex are included in this number) and ten cobalt complexes also with different types of donor nitrogen atoms and coordination numbers ranging from four up to six. Deviations of calculated bond lengths FeN and CoN from the experimental ones are randomly distributed according to the normal (Gauss) law with almost zero mean value thus indicating to the evanescence of the systematic error and with the dispersions of 0.004 and 0.001 Å^{2}, respectively. These data are obtained from the set of 180 FeN and 46 CoN internuclear separations.
Using the EHCF/SINDO1 method we performed the calculations of the electronic spectra and quadrupole splittings in Mössbauer spectra of four spincrossover Fe(II) complexes with nitrogencontaining polydentate ligands. A short description of these complexes is presented in Table . The geometries of the highspin (HS) forms of the complexes have been determined from Xray experiments (see references in Table ) and we used these geometries in our calculations. The structural data for the lowspin (LS) forms is not available for all the complexes due to the experimental difficulties with the isolation of syngle crystals. In such cases the geometries were obtained by optimization of the structures for the corresponding LS forms using the hybrid EHCF/MM method []. In all calculations of M\ßssbauer and spectral parameters we used the standard set of EHCF/SINDO1 parameters [], the Racah parameters B and C were set to 650 cm^{1} and 2400 cm^{1}, respectively, as in our previous calculations of the Fe(II) spincrossover complexes [].
The calculated dd excitation energies for all four selected complexes are presented in Table . In all cases the EHCF/SINDO1 method correctly reproduces the spin multiplicity of the ground state in accordance with experimental observations. The calculated excitation energies for the first three complexes are also in satisfactory agreement with the experimental measurements, especially taking into account that the interpretation of the experimental spectral data and assignment of the spectral bands in the visible region is usually done using the simple crystal field model for the octahedral environment and the presented measured excitation energies actually correspond to the average over the convolution of several overlapping spectral lines which are sometimes very difficult to resolve. The calculated excitation energies for the [Fe(Hpt)_{3}] complex seem to be underestimated compared to the available experimental data, especially for the LS form of the complex. However the observed transition at 18868 cm^{1} interpreted in Ref. [] as the ^{1}A_{1}®^{1}T_{1} dd transition might be interpreted based on our calculations as the transition from the ground ^{1}A_{1} state to one of the JahnTeller components of the excited ^{1}T_{2} state. Unfortunately, the details of the interpretation of the measured ligandfield spectrum are not discussed in Ref. [].
The Mössbauer spectra of the spincrossover iron complexes are with no doubt a very powerful source of information about electronic structure and nature of spin transitions in these complexes. The measured parameters of the spectra, isomer shift and quadrupole splitting, differ significantly for two spin isomers which allows to monitor the composition of the sample by decomposing the measured spectrum into HS and LS components and evaluating their relative contributions. From the theoretical point of view the calculations of the Mössbauer spectral parameters and comparing the results to the experimental data presents a unique and very sensitive way of probing the local electronic structure of the metal ion in the complex due to the extreme sensitivity of the isomer shift and quadrupole splitting to the details of the electron density in the vicinity of the iron atom and to the symmetry of the electrostatic field created by the environment (ligands). We performed the calculations of the temperature dependent quadrupole splittings for the HS and LS forms of the complexes listed in Table using the wavefunctions obtained in our EHCF/SINDO1 calculations. For the quadrupole moment of the ^{57}Fe nucleus we used the value of Q = 0.187 barn, for the radial factors á r^{3} ñ_{4p} and á r^{3} ñ_{3d} we used the values of 1.697 and 4.979, respectively, obtained in HFR atomic calculations []. No other empirical parameters were introduced in Eq.(35). The calculation results are presented in Figures  along with the available experimental data. The agreement between the calculated and experimental data for the first three complexes (Figures ) is quite remarkable. That indicates that the EHCF/SINDO1 method describes the electronic structure of the dsystem as well as the details of the electrostatic field in the vicinity of the metal atom quite accurately. As in the case with the electronic excitation spectra, the calculated quadrupole splittings for the [Fe(Hpt)_{3}] complex (especially for the HS form) are in a relatively poor numerical agreement with the experimental data, although qualitatively the results are still satisfactory. The true reason for this exceptional behaviour remains unclear, but one of the possible explanations can be related to the uncertainties in the structural data. Among the complexes considered the [Fe(Hpt)_{3}] complex is the only one which undergoes a gradual spin transition, so that both the LS and HS forms are present in the sample over all range of temperatures. To demonstrate the sensitivity of the calculated values of Mössbauer parameters to the geometry of the complex we performed additional calculations of the quadrupole splittings for the geometries optimized with the hybrid EHCF/MM method instead of experimental ones. As one can see from Figure , very slight changes in the geometry produced by the EHCF/MM optimization on the HS form result in significant variation of the calculated quadrupole splitting. In any case further studies are necessary to elucidate the correspondence between experimental and theoretical spectra and molecular geometries of the present compound.
Despite of the mentioned difficulties, the overall results of calculations strongly support the applicability of the EHCF methodology for studying the electronic structure of polyatomic TMC's for which highlevel ab initio calculations are not feasible. Moreover, the results of calculations also support the whole EHCF paradigm and its ability to describe the most important features of the electronic structure of the systems with strongly localized correlated groups of electrons on the quantitative level.
In the present paper we tried to demonstarte the feasibility of a semiempirical description of the electronic structure and properties of the Werner TMCs on a series of rather sophisticated examples. Spin active complexes (those undergoing spin transitions) of iron(II) with nitrogen containing ligands have been considered. It turned out that using a semiempirical description with the electron correlation adequately ''biult in'' to the structure of the method allows to reproduce the entire collection of the relevant experimental data ranging from the molecular geometry for the both observed spin isomers to the optical and Mössbauer spectra.
AVS thanks Professor Dr. Karl Jug for many stimulating discussions during the initial stages of the development of the EHCF/SINDO1 method and for the encourangement and strong support throughout his whole carrier. This paper is dedicated to Professor Dr. Karl Jug in recognition of his significant contribution to modern quantum chemistry and on occasion of his 65th birthday. The work is supported by the RFBR grants Nos 040332146, 040332206. Authors also thank Prof. Dr. Paul Ziesche for sending (p)reprints of his work on density matrix cumulants.




 
complex  ligand  spin transition characteristics  structural data  Ref.  
 
[Fe(mtz)_{6}]^{2+}  1methyltetrazoleN^{4} 

 []  
[Fe(ptz)_{6}]^{2+}  1propyltetrazole 

 []  
[Fe(teec)_{6}]^{2+}  1(2chloroethyl)tetrazole 

 []  
[Fe(Hpt)_{3}]^{2+}  3(2pyridyl)1,2,4triazole 

 []  

 
[Fe(mtz)_{6}]^{2+}  
HS (Xray [])  LS (EHCF/MM)  
 
transition  calculated  Exp.[]  transition  calculated  Exp.[] 
^{5}T_{2} ® ^{5}E(1)  10211  ^{1}A_{1} ® ^{1}T_{1}(1)  14929  
® ^{5}E(2)  10284  11750  ® ^{1}T_{1}(2)  14980  18200 
® ^{1}T_{1}(3)  15083  
® ^{1}T_{2}(1)  22963  
® ^{1}T_{2}(2)  23075  26600  
® ^{1}T_{2}(3)  23130  
 
 
[Fe(ptz)_{6}]^{2+}  
HS (EHCF/MM)  LS (EHCF/MM)  
 
transition  calculated  Exp.[]  transition  calculated  Exp.[] 
^{5}T_{2} ® ^{5}E(1)  8774  ^{1}A_{1} ® ^{1}T_{1}(1)  14478  
® ^{5}E(2)  8855  12590  ® ^{1}T_{1}(2)  14577  19231 
® ^{1}T_{1}(3)  14666  
® ^{1}T_{2}(1)  22512  
® ^{1}T_{2}(2)  22533  27778  
® ^{1}T_{2}(3)  22650  
 
 
[Fe(teec)_{6}]^{2+}  
HS (Xray [])  
 
transition  calculated  Exp.[]  transition  calculated  Exp. 
^{5}T_{2} ® ^{5}E(1)  6550  
® ^{5}E(2)  6972  11800  
 
 
[Fe(Hpt)_{3}]^{2+}  
HS (Xray [])  LS (Xray [])  
 
transition  calculated^{b}  Exp.[]  transition  calculated^{b}  Exp.[] 
^{5}T_{2} ® ^{5}E(1)  6625 (8896)  ^{1}A_{1} ® ^{1}T_{1}(1)  10995 (13564)  
® ^{5}E(2)  8169 (10412)  11765  ® ^{1}T_{1}(2)  11426 (13893)  18868 
® ^{1}T_{1}(3)  11668 (14613)  
® ^{1}T_{2}(1)  17993 (20892)  
® ^{1}T_{2}(2)  18778 (21999)  
® ^{1}T_{2}(3)  19448 (23148)  

^{a} Calculated excitation energies correspond to the actual symmetry of the complexes which is lower than O_{h}, hence the splittings between the components of the degenerated E and T states
^{b} numbers in brackets are the results of calculation with the geometry of the complex optimized with hybrid EHCF/MM method
^{1}Corresponding author, Email: andrei@cc.nifhi.ac.ru