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Fundamental principles for theoretical understanding and elucidation of structure and reactivity of iron-sulfur (Fe-S) FenSm (n, m=2∼8) clusters are investigated and elucidated on the theoretical and experimental grounds. To this end, the nature of chemical bonds of these clusters is investigated by three methods; (1) the spin Hamiltonian model for analysis of EPR results, (2) broken-symmetry (BS) hybrid density functional theory (HDFT) methods for full geometry optimisations and elucidations of complete active spaces (CAS) for one-electron transfers reactions and (3) beyond HDFT methods such as CAS configuration interaction (CI) and MR CI for high precision energy calculations on classical and quantum computers. Theoretical concepts revealed are applied for elucidation of the mechanism of nitrogen fixation with FeMoco (Fe7MoS9C) cluster, indicating an important role of proton-coupled (PC) one electron spin transfer (OEST) processes instead of radical reaction mechanisms. [ABSTRACT FROM AUTHOR]

Possible mechanisms for water cleavage in oxygen evolving complex (OEC) of photosystem II (PSII) have been investigated based on broken-symmetry (BS) hybrid DFT (HDFT)/def2 TZVP calculations in combination with available XRD, XFEL, EXAFS, XES and EPR results. The BS HDFT and the experimental results have provided basic concepts for understanding of chemical bonds of the CaMn4O5cluster in the catalytic site of OEC of PSII for elucidation of the mechanism of photosynthetic water cleavage. Scope and applicability of the hybrid DFT (HDFT) methods have been examined in relation to relative stabilities of possible nine intermediates such as Mn-hydroxide, Mn-oxo, Mn-peroxo, Mn-superoxo, etc., in order to understand the O–O (O–OH) bond formation in the S3and/or S4states of OEC of PSII. The relative stabilities among these intermediates are variable, depending on the weight of the Hartree–Fock exchange term of HDFT. The Mn-hydroxide, Mn-oxo and Mn-superoxo intermediates are found to be preferable in the weak, intermediate and strong electron correlation regimes, respectively. Recent different serial femtosecond X-ray (SFX) results in the S3state are investigated based on the proposed basic concepts under the assumption of different water-insertion steps for water cleavage in the Kok cycle. The observation of water insertion in the S3state is compatible with previous large-scale QM/MM results and previous theoretical proposal for the chemical equilibrium mechanism in the S3state . On the other hand, the no detection of water insertion in the S3state based on other SFX results is consistent with previous proposal of the O–OH (or O–O) bond formation in the S4state . Radical coupling and non-adiabatic one-electron transfer (NA-OET) mechanisms for the OO-bond formation are examined using the energy diagrams by QM calculations and by QM(UB3LYP)/MM calculations . Possible reaction pathways for the O–O and O–OH bond formations are also investigated based on two water-inlet pathways for oxygen evolution in OEC of PSII. Future perspectives are discussed in relation to post HDFT calculations of the energy diagrams for elucidation of the mechanism of water oxidation in OEC of PSII. [ABSTRACT FROM AUTHOR]

In this work, we show that the natural orbitals of unrestricted hybrid density functional theory (UHDFT) can be used as the active space orbitals to perform multireference (MR) calculations, for example, the density matrix renormalisation group (DMRG) and Mukherjee-type (Mk) MR coupled-cluster (CC) method. By including a sufficiently large number of these natural orbitals, full-valence (FV) active space can be identified without recourse of the expensive self-consistent procedures for DMRG-SCF. Several useful chemical indices are derived based on the occupation numbers of the natural orbitals for seamless continuation from broken-symmetry (BS) to symmetry-adapted (SA) methods. These procedures are used on 1,3-didehydrobenzene (meta-benzyne) to calculate its singlet (S)-triplet (T) gap. We compare our results to available experiments and computational results obtained by several other groups. We see our procedures as a seamless bridge between single-reference BS methods, such as UHDFT, and the SA MR methods, such as FV DMRG and MkMRCC. [ABSTRACT FROM AUTHOR]

Theoretical examinations of the ferromagnetic coupling in them-phenylene-bis-methylene molecule and its oligomer were carried out. These systems are good candidates for exchange-coupled systems to investigate strong electronic correlations. We studied effective exchange integrals (J), which indicated magnetic coupling between interacting spins in these species. First, theoretical calculations based on a broken-symmetry single-reference procedure, i.e. the UHF, UMP2, UMP4, UCCSD(T) and UB3LYP methods, were carried out with a GAUSSIAN program code under an SR wave function. From these results, theJvalue by the UHF method was largely positive because of the strong ferromagnetic spin polarisation effect. TheJvalue by the UCCSD(T) and UB3LYP methods improved an overestimation problem by correcting the dynamical electronic correlation. Next, magnetic coupling among these spins was studied using the CAS-based method of the symmetry-adapted multireference methods procedure. Thus, the UNO DMRG CASCI (UNO, unrestricted natural orbital; DMRG, density matrix renormalised group; CASCI, complete active space configuration interaction) method was mainly employed with a combination of ORCA and BLOCK program codes. DMRG CASCI calculations in valence electron counting, which included all orbitals to full valence CI, provided the most reliable result, and support the UB3LYP method for extended systems. [ABSTRACT FROM AUTHOR]

We have investigated nearsightedness of electronic matter (NEM) of finite systems on the basis of linear response function to examine theoretical foundation of contemporary computational chemistry such as quantum mechanics/molecular mechanics methods. In this study, we introduce several nearsightedness-related indices to assess the magnitude of localisability of responses for one-dimensional finite model systems. We started from two electrons' systems, which are beyond the scope of the original concept of NEM, and increased the number of electrons (N) to a hundred electrons to analyse dependency of such indices onN. In the process of construction of the indices, we analysed the factors, because of which the magnitude of nonlocal parts of linear response functions becomes small, into several ones, and extracted purely the magnitude of propagation of responses. [ABSTRACT FROM AUTHOR]

Quantum mechanical (QM)/molecular mechanics (MM) calculations by the use of a large-scale QM model (QM Model V) have been performed to elucidate hydrogen-bonding networks and proton wires for proton release pathways (PRP) of water oxidation reaction in the oxygen evolving complex (OEC) of photosystem II (PSII). Full geometry optimisations of PRP by the QM/MM model have been carried out starting from the geometry of heavy atoms determined by the recent high-resolution X-ray diffraction (XRD) experiment of PSII refined to 1.9 Å resolution. Computational results by the QM/MM calculations have elucidated the hydrogen-bonding O···O(N) and O···H distances and O(N)–H···O angles in PRP, together with the Cl–O(N) and Cl···H distances and O(N)–H···Cl angles for chloride anions. The optimised hydrogen-bonding networks are well consistent with the XRD results and available experiments such as extended X-ray absorption fine structure, showing the reliability of channel structures of OEC of PSII revealed by the XRD experiment. The QM/MM computations have elucidated possible roles of chloride anions in the OEC of PSII. The QM/MM computational results have provided useful information for understanding and explanation of accumulated mutation experiments of key amino acid residues in the OEC of PSII. Implications of the present results are discussed in relation to three steps for theoretical modelling of water oxidation in the OEC of PSII and bio-inspired working hypotheses for developments of artificial water oxidation systems by use of 3d transition-metal complexes. [ABSTRACT FROM AUTHOR]

We here formulate and implement linear response function (LRF) of the Mayer bond order (MBO), which is expected to be a new indicator to describe intrinsic chemical reactivity of molecules. We calculate LRFs of the MBOs of para-substituted benzoic acids, and compare the results with the Hammett substituent constants and computational results of acid dissociation constants that were previously reported. The results are discussed from the viewpoint of the applicability of LRF of the MBO to estimate the relative reactivity of the substituted benzoic acids. [ABSTRACT FROM AUTHOR]

In this study we investigated the photochemical production of hydrogen from water using three heterogeneous microporous ruthenium coordination polymers [Ru2(p-BDC)2X]n (p-BDC = 1,4-benzenedicarboxylate, X = Cl, Br and BF4) in the presence of multi-component systems. The order of catalytic performances is [Ru2(p-BDC)2Br]n>[Ru2(p-BDC)2BF4]n>[Ru2(p-BDC)2Cl]n. The most active catalyst, [Ru2(p-BDC)2Br]n, caused the evolution of 46.7 μmol hydrogen molecules with a turn-over number of 18.7 based on [Ru2(p-BDC)2Br]n under visible light irradiation for 4 h. We ascertained that the differences in catalytic activities originated from (1) the efficiency of the quenching of methyl-viologen radicals by [Ru2(p-BDC)2X]n and (2) the durability of the structure in the reaction. In order to examine the catalytic reaction mechanism, we performed theoretical calculations for neutral model structures [Ru2(HCOO)4X(H2O)] (X = Cl, Br), one-electron reduction model complexes [Ru2(HCOO)4X(H2O)]- and deduced intermediate model structures [H-Ru2(HCOO)4X] using broken-symmetry hybrid density functional theory methods. [ABSTRACT FROM AUTHOR]

The electronic structures of ground and excited states of [Rh2(CH3COO)4(H2O)2] in aqueous solution are studied using a density functional theory (DFT) with a time-dependent (TD) method. Up to now, several theoretical assignments and explanations of its excitation characters have been reported based on the absorption spectra. In this study, we reinvestigate its absorption spectrum by the TD-DFT approach with the polarisable continuum model in order to clarify the excitation characters of the complex, especially in the aqueous solution well. [ABSTRACT FROM AUTHOR]

We investigate for the first time the electronic structure of the biomimetic diiron complex 1 [Fe(IV)2(μ-O)2(5-Me3-TPA)2]4+ (5-Me3-TPA = tris(5-methyl-2-pyridylmethyl)amine) using broken-symmetry hybrid density functional theory method. According to previous studies, the Fe(IV)2(μ-O)2 core of intermediate Q of soluble methane monooxygenase shows antiferromagnetic coupling between local S = 2 states at each iron site. In contrast to Q, each local Fe(IV) ion of the synthetic complex does not have an S = 2 but has an S = 1 state based on the calculated geometric parameters and energetic stability. Our calculation supports the experimental result. [ABSTRACT FROM AUTHOR]

The chemical species of solvent molecule (WB) between two manganese ions in the active site of arginase is investigated based on the structural and magnetic property. The calculated magnetic coupling constant (Jab) values at UBH&HLYP level of theory are - 1.9 cm- 1 for WB = H2O and - 5.7 cm- 1 for WB = OH- , respectively. In comparison with the experimental value (Jab = ∼ - 2.0 cm- 1), H2O is the most likely candidate to WB in the initial structure. Natural orbital analyses reveal that WB plays an important role for keeping the geometry of the Mn-O(WB)-Mn core rigid rather than mediating superexchange interaction. The result indicates that WB does not have to shift to one Mn ion as an initiation of the hydrolysis reaction. [ABSTRACT FROM AUTHOR]

Mukherjee-type (Mk) state specific (SS) multi-reference (MR) coupled-cluster (CC) calculations of 1,n-didehydropolyene diradicals were carried out to elucidate singlet-triplet energy gaps via through-bond coupling between terminal radicals. Spin-unrestricted Hartree-Fock (UHF) based coupled-cluster (CC) computations of these diradicals were also performed. Comparison between symmetry-adapted MkMRCC and broken-symmetry (BS) UHF-CC computational results indicated that spin-contamination error of UHF-CC solutions was left at the SD level, although it had been thought that this error was negligible for the CC scheme in general. In order to eliminate the spin contamination error, approximate spin-projection (AP) scheme was applied for UCC, and the AP procedure indeed eliminated the error to yield good agreement with MRCC in energy. The CCD with spin-unrestricted Brueckner's orbital (UB) was also employed for these polyene diradicals, showing that large spin-contamination errors at UHF solutions are dramatically improved, and therefore AP scheme for UBD removed easily the rest of spin-contaminations. Pure- and hybrid-density functional theory (DFT) calculations of the species were also performed. Three different computational schemes for total spin angular momentums were examined for the AP correction of the hybrid DFT. The AP DFT calculations yielded the singlet-triplet energy gaps that were in good agreement with those of MRCC, AP UHF-CC and AP UB-CC. Chemical indices such as the diradical character were calculated with all these methods. Implications of the present computational results are discussed in relation to previous RMRCC calculations of diradical species and BS calculations of large exchange coupled systems. [ABSTRACT FROM AUTHOR]

Mukherjee's type of multireference coupled-cluster (MkMRCC), approximate spin-projected spin-unrestricted CC (APUCC), and AP spin-unrestricted Brueckner's (APUBD) methods were applied to didehydronated ethylene, allyl cation, cis-butadiene, and naphthalene. The focus is on descriptions of magnetic properties for these diradical species such as S-T gaps and diradical characters. Several types of orbital sets were examined as reference orbitals for MkMRCC calculations, and it was found that the change of orbital sets do not give significant impacts on computational results for these species. Comparison of MkMRCC results with APUCC and APUBD results show that these two types of methods yield similar results. These results show that the quantum spin corrected UCC and UBD methods can effectively account for both nondynamical and dynamical correlation effects that are covered by the MkMRCC methods. It was also shown that appropriately parameterized hybrid density functional theory (DFT) with AP corrections (APUDFT) calculations yielded very accurate data that qualitatively agree with those of MRCC and APUBD methods. This hierarchy of methods, MRCC, APUCC, and APUDFT, is expected to constitute a series of standard ab initio approaches towards radical systems, among which we could choose one of them, depending on the size of the systems and the required accuracy. [ABSTRACT FROM AUTHOR]

We present simple CAS-DFT approaches for degenerate and nearly degenerate systems. In this method, we do not employ any auxiliary variables, but employ the effective CASCI-DFT and CASSCF-DFT equations for the ground and excited states. Simple applications for fundamental systems such as atomic multiplet states (C, N+n, and O+n), the transition metal complex V(III)L6 (L = H2O and CO) and the Cu(II) acetate dimer Cu(II)2(Ac)4(H2O)2 are presented. The results are discussed from the viewpoint of the shapes of the potential curves of several nearly degenerate spin states, the theory of molecular magnetism and the reliability of the CAS space employed. The computational results are fully compatible with the experimental results available, and also with those of ligand field theory. [ABSTRACT FROM AUTHOR]

Inter-dimer magnetic interactions (J ab(D) ) between (BEDT-TTF) 2 + • dimers in the κ-(BEDT-TTF) 2 Cu 2 (CN) 3 crystal has been studied theoretically. In these crystals, triangular lattice structure with spin frustration has been discovered and experimental studies have been performed by several research groups. We have calculated the inter-dimer J ab(D) values by using several methods. In addition, we have analyzed the spin lattice with optimization of θ angle in classical Heisenberg spin model and with numerical diagonalization in quantum Heisenberg spin model. [ABSTRACT FROM AUTHOR]

Static molecular second hyperpolarizabilities γ of several donor-π-acceptor molecules have been examined using an ab initio molecular orbital (MO) method to evaluate the effects of substituents and molecular distortion on γ. It was found that γ values of the donor-π-acceptor compounds are affected by the degree of π electron delocalization more than the donor or acceptor strength of the substituents. Also it was found that contributions from higher order excitation process are not negligible for γ values of the donor-π-acceptor molecules. [ABSTRACT FROM AUTHOR]

The effective exchange integrals (J[sub ab(M)]) between two cation radicals of title compounds in the Heisenberg model were calculated by ab initio molecular orbital (MO) and density functional theory (DFT) methods, together with hybrid DFT methods. The J[sub ab(D)] values between two dimer mono-cation radicals were also estimated assuming J[sub ab(D)] ≈ J[sub ab(M)]/2. It is found that the spin lattice obtained by the ab initio method is square planar and linear in BEDT-TTF and BETS planes, respectively, although other previous calculations show that spin lattice in the BEDT-TTF plane is a triangular one. The J[sub ab] and overlap integrals (s[sub ab]) values by the ab initio methods were used to determine transfer integral (t[sub ab]) and Coulomb repulsion (U[sub eff]) parameters of the Hubbard model, which were compared with those of the previous results. Implications of the calculated results are discussed in relation to the spin-mediated mechanism for superconductivity. [ABSTRACT FROM AUTHOR]

The molecular second hyperpolarizabilities γ of cationic 3- or 4-substituted pyridine derivatives have been investigated. The γ values are calculated by an ab initio molecular orbital (MO) method at correlated levels including CCSD(T). The magnitude of γ for the pyridinium cation is shown to be smaller than that of pyridine. It is found that γ for the pyridinium cation changes dramatically with substitution, whereas γ for neutral pyridine does not show such large substituent effects. This remarkable substituent effect on γ of cationic pyridine derivatives is one of the special features of cationic species. [ABSTRACT FROM AUTHOR]

Potential curves and high and low spin energy gaps for radical clusters were calculated by spin polarized molecular orbital methods. Through-space effective exchange integrals (J[sub ab]) and relative energies of spin projected low spin states by post-Hartree-Fock (HF) calculation were reproduced by the hybrid density functional theory (DFT) method. The hybrid parameters that could reproduce post-HF values such as UCCSD(T)'s for each model had close relations with the instabilities of those systems. Information entropy and related chemical indices were used to estimate the magnitude of the instabilities. A magnetic effective density functional (MEDF) scheme for spin clusters was proposed for practical computation of J[sub ab] values in molecular magnetic materials. [ABSTRACT FROM AUTHOR]

Magnetic interactions between divalent chromium (Cr(II)) ions in tetra Cr(II) complex were estimated by using a magnetic effective density functional (MEDF) method. Those results were discussed in comparison with a di-Cr(II) complex to elucidate the types of magnetic interactions. Charge and spin densities and natural orbitals by MEDF were also examined in terms of the ligand effects. aligned tetrametal systems magnetic interaction effective exchange integrals Magnetic Effective Density Functional MEDF [ABSTRACT FROM AUTHOR]

The spin alignment rule for the magnetic couplings of d-π-d conjugated systems via pyrimidine has been proposed on the basis of the superexchange (SE) interaction and spin polarization (SP) effects in order to predict the sign of J ab values between transition metal ions. This rule shows that the subtle balance of σ- and π- type SE interaction and SP effects determines the magnetic interaction of these systems. M(II) 2 -pyrimidine effective exchange interaction ( J ab ) superexchange (SE) interaction spin polarization (SP) effect [ABSTRACT FROM AUTHOR]

Cooperation of antiferromagnetism and superconductivity is achieved and reported by Kobayashi and his co-workers. For the BETS related crystals, κ-(BETS) 2 FeCl 4 (BETS=bis(ethylenedithio)tetraselenafulvalene), effective exchange integrals ( J ab ) between two BETS + molecules (Step I) and between BETS + and FeCl 4 - molecules (Step II) are very important to reveal superconductivity and successfully evaluated in our treatments. organic superconductor effective exchange integral BETS ab initio MO DFT [ABSTRACT FROM AUTHOR]

Phase diagrams of theoretical models for CuO 2 plane and CuO chains are presented by using a two-band model. Field-induced superconductivity Two-band model High-Tc superconductivity [ABSTRACT FROM AUTHOR]

We have firstly developed the ab initio linear combination of gaussian type orbital (LCGTO) program of generalized spin orbital (GSO) density functional theory (DFT) and applied it for multicenter metal clusters with noncollinear spin structures. It was found that two- and three-dimensional spin states becomes ground-states for Cr 3 (D 3h ) and CR 4 (T d ) respectively, indicating importance of GSO treatment of DFT for these clusters. Generalized spin orbital density functional theory multicenter metal clusters noncollinear molecular magnetism [ABSTRACT FROM AUTHOR]

Model clusters (NH 3 ) 4 +2 and (NM 3 -H 2 NO·) 4 +2 are introduced as simple models for a pure organic conductor. Then theoretical calculations of the total energies of ab initio solution, in which the spin or charge density are arranged alternately, and intermolecular magnetic interactions are carried out by ab initio MO-DFT method. The result shows that these model clusters are efficient for ab initio investigation of the electronic properties of organomagnetic conductors. ab initio MO calculation pure organomagnetic conductors effective exchange integrals cluster model [ABSTRACT FROM AUTHOR]

The transfer matrices of AgO 2 plane and Ag 2 O 6 chain are estimated by using the restricted open-shell B3LYP method. For AgO 2 plane, the charge transfer energy Δ between two sites is compared with that of CuO 2 plane. For Ag 2 O 6 chain, the band structures are estimated by changing value of Δ, and the conductivity of this chain is discussed in relation to the superconductivity from the analysis of band structures. silver oxide transfer energy superconductivity [ABSTRACT FROM AUTHOR]

SDW and CDW states of [-O-M-O-M-O-M-O-M-](M=Cu, Ag)4 site periodic model were investigated. Our theoretical attempt is to elucidate the difference among group 11 metals; Cu, Ag, from the view point of stability on SDW/CDW states, and leads to prospect of their physical properties. SDW CDW charge fluctuation periodic model Cu and Ag oxides [ABSTRACT FROM AUTHOR]

AgPb 6 CO 9 crystal has been simulated by means of molecular dynamics calculational method. A new simple interatomics potential model for this crystal was developed so that the crystal structure remains quite stable for all the simulation time steps. By analysis of the mean squared displacement, we studied the atomic motions in this crystal structure in detail. Molecular Dynamics superconductivity [ABSTRACT FROM AUTHOR]

The detailed theoretical studies for TCNQ/TCNQ -· salts, which possess ferromagnetic hysteresis at relative high temperature, were carried out. The calculations based on ab initio molecular orbital (MO) and density functional (DFT) theories were carried out to evaluate the intermolecular effective exchange integrals and transfer integrals. In addition, we employed the "hybrid DFT method" based on instability of chemical bonds and successfully studied their spin densities. From the results it was found the remarkable singlet dimer structure is dominant. In addition, quasi-one-dimensional structure was found because of nonzero charge transfer. [ABSTRACT FROM AUTHOR]

The unrestricted Hartree-Fock (UHF) and hybrid density functional theory (HDFT) calculations have been carried out for manganese oxides. In order to elucidate magnetic properties of the species, the effective exchange integrals ( J ab ) have been obtained by the total energy difference between the highest and lowest spin states. The natural orbital analysis has also been performed for elucidation of symmetry and occupation numbers of the magnetic orbitals. [ABSTRACT FROM AUTHOR]

The effective exchange integrals for the naked Cr(II) tetramer are calculated by using post-Hartree-Fock and the improved hybrid density functional method. The calculated values are examined in comparison with naked Cr(II) dimer. The naked Cr(II) tetramer that is arranged similar to [Cr(II) 4 (DpyF) 4 Cl 2 ]Cl 2 also examined in terms of magnetic interactions. [ABSTRACT FROM AUTHOR]