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The modern project management environment is becoming more and more volatile under the influence of exogenous and endogenous factors. The key factor in this context is motivation, which has the potential to increase its application in project management. The aim of the article is to analyze the motivation strategies that can be implemented by the project management office to increase the success of projects. The importance of motivation in the context of project management is considered, with an emphasis on how motivational strategies can affect the productivity and effectiveness of teams. The importance of the project management office in the development of team motivation to achieve success in project management in conditions of increased volatility of the business environment is emphasized. According to McKinsey, 37% of situations leading to the failure of a project are due to changes in its goals, which directly or indirectly affects motivation. Various methods of motivation used by the project management office, including intrinsic and extrinsic motivation, and their impact on the behavior of employees are considered. Attention is paid to the psychological aspects of motivation, the role of cultural and individual differences. Particular attention is paid to the use of metrics for quantitative project management, which provides a more structured and objective approach to evaluating and improving various aspects of the project, including motivation. An analysis of the importance of creating a favorable environment for motivation and the use of a strategic approach to the implementation of motivational strategies is carried out. The proposed research can be useful for professional analysts and researchers, companies, non-profit organizations and authorities who seek to strengthen the motivation component of the project management office. Prospects for further research include the development of a KPI system taking into account the main organizational goals and, on the basis of their construction, a motivation system by the project management office.

The time-independent eigenstate-free Raman wavefunction approach for calculating anharmonic vibronic spectra has been extended for the calculation of Herzberg–Teller contributions on the basis of an n-mode expansion of the transition electric dipole moment surface. This allows for the efficient simulation of Franck–Condon dark vibronic spectra. In addition, vibrational angular momentum terms have been implemented into this formalism, as they are important for an accurate description of vibrational wavefunctions spanning double-well potentials. This approach has been used to compute the FC-forbidden vibronic spectrum of the n → π* transition of formaldehyde based on a potential energy and transition dipole moment surfaces obtained from explicitly correlated multi-reference configuration interaction calculations. An extensive analysis of the resulting vibronic structure is provided, which allows for a detailed assignment and interpretation of the experimental spectrum. [ABSTRACT FROM AUTHOR]

The transitions in a series of prototypical complexes in depth were studied in combination of resonance Raman spectroscopy and quantum chemistry. These studies have defined the information content of absorption and rR spectroscopy for the compounds investigated and such transitions occur when the ligands have significant open-shell character.

Vibrational configuration interaction theory is a common method for calculating vibrational levels and associated IR and Raman spectra of small and medium-sized molecules. When combined with appropriate configuration selection procedures, the method allows the treatment of configuration spaces with up to 1010 configurations. In general, this approach pursues the construction of the eigenstates with significant contributions of physically relevant configurations. The corresponding eigenfunctions are evaluated in the subspace of selected configurations. However, it can easily reach the dimension which is not tractable for conventional eigenvalue solvers. Although Davidson and Lanczos methods are the methods of choice for calculating exterior eigenvalues, they usually fall into stagnation when applied to interior states. The latter are commonly treated by the Jacobi-Davidson method. This approach in conjunction with matrix factorization for solving the correction equation (CE) is prohibitive for larger problems, and it has limited efficiency if the solution of the CE is based on Krylov's subspace algorithms. We propose an iterative subspace method that targets the eigenvectors with significant contributions to a given reference vector and is based on the optimality condition for the residual norm corresponding to the error in the solution vector. The subspace extraction and expansion are modified according to these principles which allow very efficient calculation of interior vibrational states with a strong multireference character in different vibrational structure problems. The convergence behavior of the method and its performance in comparison with the aforementioned algorithms are investigated in a set of benchmark calculations. [ABSTRACT FROM AUTHOR]

For larger molecules, the computational demands of configuration selective vibrational configuration interaction theory (cs‐VCI) are usually dominated by the configuration selection process, which commonly is based on second order vibrational Møller‐Plesset perturbation (VMP2) theory. Here we present two techniques, which lead to substantial accelerations of such calculations while retaining the desired high accuracy of the final results. The first one introduces the concept of configuration classes, which allows for a highly efficient exploitation of the analogs of the Slater‐Condon rules in vibrational structure calculations with large correlation spaces. The second approach uses a VMP2 like vector for augmenting the targeted vibrational wavefunction within the selection of configurations and thus avoids any intermediate diagonalization steps. The underlying theory is outlined and benchmark calculations are provided for highly correlated vibrational states of several molecules. [ABSTRACT FROM AUTHOR]

The calculation of vibronic spectra and resonance Raman intensities can be performed on the basis of the Raman wavefunction (RWF) formalism. In general, the well-known sum-over-states (SOS) and time-dependent methods can be applied for calculating the RWF. We present an alternative route in which the RWF is determined pointwise in a spectral range on the basis of the inhomogeneous Schrödinger equation using an iterative subspace method, in which explicit state-by-state calculations of vibrational eigenstates are bypassed. We study this approach within the framework of vibrational configuration interaction theory in conjunction with high-level electronic structure calculations for the multidimensional Born-Oppenheimer potential energy surface. The method benefits from an implicit account of interference effects between vibrational states, so that its computational cost correlates with the required resolution in the spectra. The accuracy and efficiency of the method with respect to comparable SOS calculations are tested for the simulation of the photoelectron spectra of ClO2, HS2-, ZnOH-, and Zn(H2O)+. [ABSTRACT FROM AUTHOR]

The time-independent eigenstate-free Raman wavefunction approach, which is based on the vibrational inhomogeneous Schrödinger equation (ISE), is a promising alternative to the well-known sum-over-states and time-dependent methods for the calculation of optical bandshapes and resonance Raman intensities. The solution of the ISE, the Raman wavefunction (RWF), can be obtained within a vibrational configuration interaction (VCI) ansatz. In this method, the optimisation of vibrational modals can provide a better convergence of the calculated RWF towards the full VCI limit. For this purpose, the variational principle for the ISE problem is formulated. The variational condition for the modal functions leads to the complex inhomogeneous vibrational self-consistent field equations. The key aspects of the implementation of this approach are discussed. The performance of this method in comparison with the conventional scheme involving VCI bases of different quality is illustrated for the X ~ 2 A 2 ⟵ X ~ 1 A 1 transition in furan. The quantitative effects due to the modal optimisation are found to be relatively small, as compared to the impact of moderate extension of the configuration basis. [ABSTRACT FROM AUTHOR]

In this work, an improved method for the efficient automatic simulation of optical band shapes and resonance Raman (rR) intensities within the 'independent mode displaced harmonic oscillator' is described. Despite the relative simplicity of this model, it is able to account for the intensity distribution in absorption (ABS), fluorescence, and rR spectra corresponding to strongly dipole allowed electronic transitions with high accuracy. In order to include temperature-induced effects, we propose a simple extension of the time dependent wavepacket formalism developed by Heller which enables one to derive analytical expressions for the intensities of hot bands in ABS and rR spectra from the dependence of the wavepacket evolution on its initial coordinate. We have also greatly optimized the computational procedures for numerical integration of complicated oscillating integrals. This is important for efficient simulations of higher-order rR spectra and excitation profiles, as well as for the fitting of experimental spectra of large molecules. In particular, the multimode damping mechanism is taken into account for efficient reduction of the upper time limit in the numerical integration. Excited state energy gradient as well as excited state geometry optimization calculations are employed in order to determine excited state dimensionless normal coordinate displacements. The gradient techniques are highly cost-effective provided that analytical excited state derivatives with respect to nuclear displacements are available. Through comparison with experimental spectra of some representative molecules, we illustrate that the gradient techniques can even outperform the geometry optimization method if the harmonic approximation becomes inadequate. [ABSTRACT FROM AUTHOR]

In this paper, we present the implementation of efficient approximations to time-dependent density functional theory (TDDFT) within the Tamm-Dancoff approximation (TDA) for hybrid density functionals. For the calculation of the TDDFT/TDA excitation energies and analytical gradients, we combine the resolution of identity (RI-J) algorithm for the computation of the Coulomb terms and the recently introduced 'chain of spheres exchange' (COSX) algorithm for the calculation of the exchange terms. It is shown that for extended basis sets, the RIJCOSX approximation leads to speedups of up to 2 orders of magnitude compared to traditional methods, as demonstrated for hydrocarbon chains. The accuracy of the adiabatic transition energies, excited state structures, and vibrational frequencies is assessed on a set of 27 excited states for 25 molecules with the configuration interaction singles and hybrid TDDFT/TDA methods using various basis sets. Compared to the canonical values, the typical error in transition energies is of the order of 0.01 eV. Similar to the ground-state results, excited state equilibrium geometries differ by less than 0.3 pm in the bond distances and 0.5° in the bond angles from the canonical values. The typical error in the calculated excited state normal coordinate displacements is of the order of 0.01, and relative error in the calculated excited state vibrational frequencies is less than 1%. The errors introduced by the RIJCOSX approximation are, thus, insignificant compared to the errors related to the approximate nature of the TDDFT methods and basis set truncation. For TDDFT/TDA energy and gradient calculations on Ag-TB2-helicate (156 atoms, 2732 basis functions), it is demonstrated that the COSX algorithm parallelizes almost perfectly (speedup ∼26-29 for 30 processors). The exchange-correlation terms also parallelize well (speedup ∼27-29 for 30 processors). The solution of the Z-vector equations shows a speedup of ∼24 on 30 processors. The parallelization efficiency for the Coulomb terms can be somewhat smaller (speedup ∼15-25 for 30 processors), but their contribution to the total calculation time is small. Thus, the parallel program completes a Becke3-Lee-Yang-Parr energy and gradient calculation on the Ag-TB2-helicate in less than 4 h on 30 processors. We also present the necessary extension of the Lagrangian formalism, which enables the calculation of the TDDFT excited state properties in the frozen-core approximation. The algorithms described in this work are implemented into the ORCA electronic structure system. [ABSTRACT FROM AUTHOR]

A general method for the simulation of absorption (ABS) and fluorescence band shapes, resonance-Raman (rR) spectra, and excitation profiles based on the time-dependent theory of Heller is discussed. The following improvements to Heller’s theory have been made: (a) derivation of new recurrence relations for the time-dependent wave packet overlap in the case of frequency changes between the ground and electronically excited states, (b) a new series expansion that gives insight into the nature of Savin’s preresonance approximation, (c) incorporation of inhomogeneous broadening effects into the formalism at no additional computational cost, and (d) derivation of a new and simple short-time dynamics based equation for the Stokes shift that remains valid in the case of partially resolved vibrational structure. Our implementation of the time-dependent theory for the fitting of experimental spectra and the simulation of model spectra as well as the quantum mechanical calculation of the model parameters is discussed. The implementation covers all electronic structure approaches which are able to deliver ground- and excited-state energies and transition dipole moments. The technique becomes highly efficient if analytic gradients for the excited-state surface are available. In this case, the computational cost for the simultaneous prediction of ABS, fluorescence, and rR spectra is equal to that of a single excited-state geometry optimization step while the limitations of the short-time dynamics approximation are completely avoided. As a test case we discuss the well-known case of the strongly allowed 1 1Ag→1 1Bu transition in 1,3,5 trans-hexatriene in detail using method ranging from simple single-reference treatments to elaborate multireference electronic structure approaches. At the highest computational level, the computed spectra show the best agreement that has so far been obtained with quantum chemical methods for this problem. [ABSTRACT FROM AUTHOR]

A systematic study of 12 ferric and ferrous K[beta] X-ray emission spectra (XES) was conducted to experimentally assess and quantitatively evaluate the factors that contributed to the K[beta] main line and the valence to core region of the spectra. The findings offer an important calibration for future applications to iron active sites in biological and chemical catalysis and potential applications to compound II heme derivatives.

Synchrotron-based nuclear resonance vibrational scattering (NRVS) is used for characterizing the Fe-N stretching frequency of an Fe(V)-nitrido complex and its Fe(III)-azide precursor. The NRVS spectra are successfully fitted and simulated by using the density functional theory (DFT) approach.

The X ̃ 2 A 2 ← X ̃ 1 A1 photoelectron spectrum of furan has been studied by a time-independent eigenstate-free Raman wave function approach based on multi-dimensional potential energy surfaces obtained from explicitly correlated distinguishable clusters calculations. Individual vibronic transitions with the most significant Franck-Condon factors were determined by our recently developed residual-based algorithm for the calculation of eigenpairs in conjunction with the formalism of contracted invariant Krylov subspaces. The account of anharmonic and temperature effects allowed us to explain most bands in an experimental high-resolution zero kinetic energy photoelectron spectrum. This led to the reassignment of many spectral features, as well as a refined interpretation of the intensity mechanism for the corresponding transitions. [ABSTRACT FROM AUTHOR]

In this paper, various implementations of the analytic Hartree–Fock and hybrid density functional energy second derivatives are studied. An approximation-free four-centre implementation is presented, and its accuracy is rigorously analysed in terms of self-consistent field (SCF), coupled-perturbed SCF (CP-SCF) convergence and prescreening criteria. The CP-SCF residual norm convergence threshold turns out to be the most important of these. Final choices of convergence thresholds are made such that an accuracy of the vibrational frequencies of better than 5 cm−1compared to the numerical noise-free results is obtained, even for the highly sensitive low frequencies (<100–200 cm−1). The effects of the choice of numerical grid for density functional exchange–correlation integrations are studied and various weight derivative schemes are analysed in detail. In the second step of the work, approximations are introduced in order to speed up the computation without compromising its accuracy. To this end, the accuracy and efficiency of the resolution of identity approximation for the Coulomb terms and the semi-numerical chain of spheres approximation to the exchange terms are carefully analysed. It is shown that the largest performance improvements are realised if either Hartree–Fock exchange is absent (pure density functionals) and otherwise, if the exchange terms in the CP-SCF step of the calculation are approximated by the COSX method in conjunction with a small integration grid. Default values for all the involved truncation parameters are suggested. For vancomycine (176 atoms and 3593 basis functions), the RIJCOSX Hessian calculation with the B3LYP functional and the def2-TZVP basis set takes ∼3 days using 16 Intel® Xeon® 2.60GHz processors with the COSX algorithm having a net parallelisation scaling of 11.9 which is at least ∼20 times faster than the calculation without the RIJCOSX approximation. [ABSTRACT FROM PUBLISHER]

The three square planar complexes [N(η-Bu)4]2[CIIL2]·4CH3CN (S=2; 1), [N(n-Bu)4][CoIIIL2] (S=1; 3), and [N(n-Bu)4]2[RhIIL2]·4CH3CN (S= ½; 4) have been prepared and structurally characterized by X-ray crystallography (L - represents the 3,6-bis(trimethylsily)benzene-1 ,2-dithiolate(2-)). Aerial oxidation of CH2CI2 solutions of 1 produced purple crystals of [N(n-Bu)4][CrVOL2]·2CH2CI2 (S=1/2; 2), the structure of which has also been determined by X-ray crystallography. The electro- and magnetochemistry of all species has been studied. Their electronic structures have been experimentally investigated by recording their electronic and EPR spectra; solid-state temperature-dependent magnetic susceptibility measurements have been performed. Density functional theoretical calculations at the ZORA-B3LYP level have been carded out for all four species in order to obtain a better understanding of the electronic structure of the square planar complexes and the one-electron reduced [RhIL2]3- species. The reactivity( of the latter with CH3I has been studied, and two products have been identified: cis-[RhIIll2·(L")(L')] and cis-[RhIII (CH3)(l)(L")(L')where (L')- represents the S-methylated monoanion of L2- and (L") is the neutral, twice-S-methylated form of (L)2-. [ABSTRACT FROM AUTHOR]

Twelve structural models for the S2state of the oxygen-evolving complex (OEC) of photosystem II are evaluated in terms of their magnetic properties. The set includes ten models based on the ‘fused twist’ core topology derived by polarized EXAFS spectra and two related models proposed in recent mechanistic investigations. Optimized geometries and spin population analyses suggest that Mn(iii), which is most often identified with the manganese ion at site D, is always associated with a penta-coordinate environment, unless a chloride is directly ligated to the metal. Exchange coupling constants were determined by broken-symmetry density functional theory calculations and the complete spectrum of magnetic sublevels was obtained by direct diagonalization of the Heisenberg Hamiltonian. Seven models display a doublet ground state and are considered spectroscopic models for the ground state corresponding to the multiline signal (MLS) of the S2state of the OEC, whereas the remaining five models display a sextet ground state and could be related to the g= 4.1 signal of the S2state. It is found that the sign of the exchange coupling constant between the Mn centres at positions A and B of the cluster is directly related to the ground state multiplicity, implying that interconversion between the doublet and sextet can be induced by only small structural perturbations. The recently proposed quantum chemical method for the calculation of 55Mn hyperfine coupling constants is subsequently applied to the S2MLS state models and the quantities that enter into the individual steps of the procedure (site-spin expectation values, intrinsic site isotropic hyperfine parameters and projected 55Mn isotropic hyperfine constants) are analyzed and discussed in detail with respect to the structural and electronic features of each model. The current approach performs promisingly. It reacts sensitively to structural distortions and hence may be able to distinguish between different structural proposals. Thus it emerges as a useful contributor to the ongoing efforts that aim at establishing correlations between the body of spectroscopic data available for the various Sistates of the OEC and their actual geometric features. [ABSTRACT FROM AUTHOR]

The characterization of high-valent iron species is of interest due to their relevance to biological reaction mechanisms. Recently, we have synthesized and characterized an [Fe(V)-nitrido-cyclam-acetato]+ complex, which has been characterized by Mössbauer, magnetic susceptibility data, and XAS spectroscopies combined with DFT calculations (Aliaga-Alcade, N.; DeBeer George, S.; Bill, E.; Wieghardt, K.; Neese, F. Angew. Chem., Int. Ed. 2005, 44, 2908-2912). The results of this study indicated that the [Fe(V)-nitrido- cyclam-acetato)+ complex is an unusual d³ system with a nearly orbitally degenerate S = 1/2 ground state. Although the calculations predicted fairly different Fe-N stretching frequencies for the S = 1/2 and the competing S = 1/2 ground states, a direct experimental determination of this important fingerprint quantity was missing. Here we apply synchrotron-based nuclear resonance vibrational scattering (NRVS) to characterize the Fe-N stretching frequency of an Fe(V)-nitrido complex and its Fe(III)-azide precursor. The NRVS data show a new isolated band at 864 cm-1 in the Fe(V)-nitrido complex that is absent in the precursor. The NRVS spectra are fit and simulated using a DFT approach, and the new feature is unambiguously assigned to a Fe(V)-N stretch. The calculated Fe-N stretching frequency is too high by ~75 cm-1. Anharmonic contributions to the Fe-N stretching frequency have been evaluated and have been found to be small (-5.5 cm-1). The NRVS data provided a unique opportunity to obtain this vibrational information, which had eluded characterization by more traditional vibrational spectroscopies. [ABSTRACT FROM AUTHOR]

A series of transition metal complexes involving non-innocent o-dithiolene and o-phenylenediamine ligands has been characterized in detail by various spectroscopic methods like magnetic circular dichroism (MCD), absorption (abs), resonance Raman (rR), electron paramagnetic resonance (EPR), and sulfur K-edge X-ray absorption spectroscopies. A computational model for the electronic structure of the complexes is then proposed based on the density functional theory (DFT) or ab-initio methods, which can successfully account for the observed trends in the experimental spectra (MCD, rR, and abs) of the complexes. Based on these studies, the innocent vs non-innocent nature of the ligands in a given transition metal complex is found to be dependent on the position of the central metal ion in the periodic table, its effective nuclear charge in interplay with relativistic effects. [ABSTRACT FROM AUTHOR]

Vibrational configuration interaction calculations (VCI) have been performed to study the impact of the nature of the underlying coordinate systems, i.e., canonical vs localized normal coordinates, on accurate vibrational structure calculations for non-Abelian molecules. Once the correlation space is represented by real-valued primitive Hartree products, the assignment of vibrational states beyond the fundamentals is usually a tedious task and is further complicated by the use of non-symmetry-adapted coordinates. Our recently presented approach based on sparse grid integration of overlap integrals of the VCI wave function with the corresponding harmonic oscillator wave function has been used to determine and assign all fundamentals and vibrational overtones of allene and its deuterated isotopologue. Excellent agreement with available experimental results is observed in all cases.

The X̃  2 A 2 ←X̃  1 A 1 photoelectron spectrum of furan has been studied by a time-independent eigenstate-free Raman wave function approach based on multi-dimensional potential energy surfaces obtained from explicitly correlated distinguishable clusters calculations. Individual vibronic transitions with the most significant Franck-Condon factors were determined by our recently developed residual-based algorithm for the calculation of eigenpairs in conjunction with the formalism of contracted invariant Krylov subspaces. The account of anharmonic and temperature effects allowed us to explain most bands in an experimental high-resolution zero kinetic energy photoelectron spectrum. This led to the reassignment of many spectral features, as well as a refined interpretation of the intensity mechanism for the corresponding transitions.

Due to a low-lying fragmentation channel, the X̃ 2 B 1 ← X̃ 1 A 1 photoelectron spectrum of difluoromethane is dominated by strong anharmonicity effects. We have used a time-independent eigenstate-free Raman wave function approach (RWF) to calculate the entire spectrum. Vibronic transitions with the most significant Franck-Condon factors were determined by employing our recently developed residual-based algorithm for the calculation of eigenpairs (RACE). An analysis of the factors controlling the accuracy of the predicted band shape is provided. The calculated spectrum is in very close agreement with experimental results.

The three square planar complexes [N(n-Bu)(4)](2)[Cr(II)L(2)] x 4 CH(3)CN (S = 2; 1), [N(n-Bu)(4)][Co(III)L(2)] (S = 1; 3), and [N(n-Bu)(4)](2)[Rh(II)L(2)] x 4 CH(3)CN (S = 1/2; 4) have been prepared and structurally characterized by X-ray crystallography (L(2-) represents the 3,6-bis(trimethylsilyl)benzene-1,2-dithiolate(2-)). Aerial oxidation of CH(2)Cl(2) solutions of 1 produced purple crystals of [N(n-Bu)(4)][Cr(V)OL(2)] x 2 CH(2)Cl(2) (S = 1/2; 2), the structure of which has also been determined by X-ray crystallography. The electro- and magnetochemistry of all species has been studied. Their electronic structures have been experimentally investigated by recording their electronic and EPR spectra; solid-state temperature-dependent magnetic susceptibility measurements have been performed. Density functional theoretical calculations at the ZORA-B3LYP level have been carried out for all four species in order to obtain a better understanding of the electronic structure of the square planar complexes and the one-electron reduced [Rh(I)L(2)](3-) species. The reactivity of the latter with CH(3)I has been studied, and two products have been identified: cis-[Rh(III)I(2)(L'')(L')] and cis-[Rh(III)(CH(3))(I)(L'')(L')] where (L')(-) represents the S-methylated monoanion of L(2-) and (L'')(0) is the neutral, twice-S-methylated form of (L)(2-).

Twelve structural models for the S(2) state of the oxygen-evolving complex (OEC) of photosystem II are evaluated in terms of their magnetic properties. The set includes ten models based on the 'fused twist' core topology derived by polarized EXAFS spectra and two related models proposed in recent mechanistic investigations. Optimized geometries and spin population analyses suggest that Mn(iii), which is most often identified with the manganese ion at site D, is always associated with a penta-coordinate environment, unless a chloride is directly ligated to the metal. Exchange coupling constants were determined by broken-symmetry density functional theory calculations and the complete spectrum of magnetic sublevels was obtained by direct diagonalization of the Heisenberg Hamiltonian. Seven models display a doublet ground state and are considered spectroscopic models for the ground state corresponding to the multiline signal (MLS) of the S(2) state of the OEC, whereas the remaining five models display a sextet ground state and could be related to the g = 4.1 signal of the S(2) state. It is found that the sign of the exchange coupling constant between the Mn centres at positions A and B of the cluster is directly related to the ground state multiplicity, implying that interconversion between the doublet and sextet can be induced by only small structural perturbations. The recently proposed quantum chemical method for the calculation of (55)Mn hyperfine coupling constants is subsequently applied to the S(2) MLS state models and the quantities that enter into the individual steps of the procedure (site-spin expectation values, intrinsic site isotropic hyperfine parameters and projected (55)Mn isotropic hyperfine constants) are analyzed and discussed in detail with respect to the structural and electronic features of each model. The current approach performs promisingly. It reacts sensitively to structural distortions and hence may be able to distinguish between different structural proposals. Thus it emerges as a useful contributor to the ongoing efforts that aim at establishing correlations between the body of spectroscopic data available for the various S(i) states of the OEC and their actual geometric features.

The reliable correlation of structural features and magnetic or spectroscopic properties of oligonuclear transition-metal complexes is a critical requirement both for research into innovative magnetic materials and for elucidating the structure and function of many metalloenzymes. We have developed a novel method that for the first time enables the extraction of hyperfine coupling constants (HFCs) from broken-symmetry density functional theory (BS-DFT) calculations on clusters. Using the geometry-optimized tetranuclear manganese complex [Mn(4)O(6)(bpy)(6)](4+/3+) as a model, we first examine in detail the calculation of exchange coupling constants J through the BS-DFT approach. Complications arising from the indeterminacy of experimentally fitted J constants are identified and analyzed. It is found that only the energy levels derived from Hamiltonian diagonalization are a physically meaningful basis for comparing theory and experiment. Subsequently, the proposed theoretical scheme is applied to the calculation of (55)Mn HFCs of the Mn(III,IV,IV,IV) state of the complex, which is similar to the S(2) state of the oxygen-evolving complex (OEC) in photosystem II of oxygenic photosynthesis. The new approach performs reliably and accurately, and yields calculated HFCs that can be directly compared with experimental data. Finally, we carefully examine the dependence of HFC on the J value and draw attention to the sensitivity of the calculated values to the exchange coupling parameters. The proposed strategy extends naturally to hetero-oligonuclear clusters of arbitrary shape and nuclearity, and hence is of general validity and usefulness in the study of magnetic metal clusters. The successful application of the new approach presented here is a first step in the effort to establish correlations between the available spectroscopic information and the structural features of complex metalloenzymes like OEC.

Iron K-edge X-ray absorption pre-edge features have been calculated using a time-dependent density functional approach. The influence of functional, solvation, and relativistic effects on the calculated energies and intensities has been examined by correlation of the calculated parameters to experimental data on a series of 10 iron model complexes, which span a range of high-spin and low-spin ferrous and ferric complexes in O(h) to T(d) geometries. Both quadrupole and dipole contributions to the spectra have been calculated. We find that good agreement between theory and experiment is obtained by using the BP86 functional with the CP(PPP) basis set on the Fe and TZVP one of the remaining atoms. Inclusion of solvation yields a small improvement in the calculated energies. However, the inclusion of scalar relativistic effects did not yield any improved correlation with experiment. The use of these methods to uniquely assign individual spectral transitions and to examine experimental contributions to backbonding is discussed.