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First principle calculations of the effective exchange integrals ( J) in the Heisenberg model for diradical species are presented for both symmetry-adapted multi-reference (MR) and single-reference broken-symmetry (BS) methods. The Mukherjee-type state-specific MR coupled cluster singles and doubles (MkCCSD) method with several different reference orbitals including BS natural orbitals is used to calculate the singlet-triplet energy gaps (S-T energy gap or 2 J) and diradical characters for the antiaromatic molecules [ a cyclopropenyl anion (CPA), b cyclobutadiene (CBD), and c cyclopentadienyl cation (CPC)], the cyclobutadiene derivatives with polar substitutents [ d aminocyclobutadiene (ACBD), e formylcyclobutadiene (FCBD), and f 1-amino-2-formyl-cyclobutadiene (AFCBD)] and finally the cyclobutadine derivatives with radical substitutents [ g 1,2-bis(methylene)cyclobutadiene (1,2-BMCBD) and h 1,3-bis(methylene)cyclobutadiene (1,3-BMCBD)]. For the BS methods, the spin-unrestricted Hartree-Fock based CCSD (UHF-CCSD), the CCD with the spin-unrestricted Brueckner determinant (UBD), and BS density functional theory (UDFT) computations are performed. Comparison between MkCCSD and the UHF-CCSD results indicates that spin-contamination of UHF-CCSD solutions still remains. In comparison with UHF-CCSD, the UBD results show that spin-contamination involved in BS solutions is greatly suppressed. To eliminate the spin contamination, an approximate spin-projection (AP) scheme is applied to the BS solutions. The AP procedure with the use of the expectation value of the total-spin operator corresponding to UHF-CCSD and UBD results yields good agreement with the MkCCSD results. As for the AP correction of the UDFT methods, three different computational schemes for predicting the expectation value of the total-spin operator are examined. Systematic comparisons between these methods are presented for the S-T energy gaps (2 J). Implications of the present computational results have been discussed in relation to the design of magnetic oligomers and polymers. [ABSTRACT FROM AUTHOR]

Abstract: The effective exchange integrals (J(HB)) of the Heisenberg spin model have been evaluated by using the ab initio MO and based on Hartree–Fock (HF) and density functional theory (DFT) for organic magnetic metals, the (EDT-TTFVO)2FeBr4 crystal based on the X-ray crystallographic structures at 113K. In order to study the magnetic properties, we proposed some of the pairs, where the direct (d–d) and indirect (d–π–d) magnetic couplings between Fe(III) d-spins (S =5/2) without/with π-dimer spins (S =1/2) were calculated, respectively. The effective exchange integrals were evaluated by using UB3LYP method, and principal J values were 0.5, −0.1 and 0.4K. From these results, it is found that there were three dimensional spin arrangements of Fe(III) d-spins. The Quantum Monte Carlo (QMC) simulations had been carried out with our calculated J values to evaluate the magnetic susceptibility for this molecular crystal, reproducing the experimental tendency. [Copyright &y& Elsevier]

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]

Lately, liquid phase catalytic reactions over quasi heterogeneous catalysts, whose active site are stabilized by polymers, dendorimers, and soforth, are reported. It is well known that the heterojunction between the metal clusters and the metal oxide supports is important factor for the generation of catalytic activities of the heterogeneous catalysts, such as the metal oxide-supported Au catalysts. However, there is no metal oxide supports in the quasi heterogeneous catalysts. Therefore, we investigate the difference between the bare cluster and the polymer stabilized clusters using hybrid density functional theory. From the calculation results, it was found that the charge transfer interaction between the stabilizing molecules and the cluster plays an important role to exhibit the catalytic activities of the quasi heterogeneous catalysts. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

In this article, we performed spin-unrestricted Hartree Fock (UHF) based coupled-cluster (CC) and Bruckner CC double calculations of methylene, fluorine, H&bond;He&bond;H diradical, and ozone. Mukherjee-type (Mk) Multireference coupled-cluster (MRCC) calculations of these species were also conducted by the use of UHF natural orbital (UNO) and UHF localized natural orbitals (ULO) as reference orbitals to elucidate size-consistent error. Comparing calculated results of spin-unrestricted Hartree-Fock based coupled-cluster (UCC) and spin-unrestricted Bruckner-Double (UBD) with those of MkMRCC, it was found that spin-contamination errors of UCC and UBD solutions still remained, though this type of errors for these solutions was considered to be negligible in general. The comparison also revealed scope and applicability of approximate spin-projection (AP) scheme for UCC and UBD, indicating that the AP procedure corrected spin-contamination errors to yield good agreement with MRCC in energy. We also performed pure- and hybrid- density functional theory (DFT) calculations of the same systems, and those calculations provided reasonable singlet-triplet energy gaps, which were qualitatively good agreement with those of MRCC, AP-UCC, and AP-UBD calculations. Implications of the computational results are discussed in relation to applicability of hybrid DFT to large superexchange coupled systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

Broken-symmetry (BS) and approximate spin-projected (AP) BS hybrid density functional theory (DFT) calculations were performed to elucidate possible mechanisms of hydroxylation reactions of methane and alkanes with soluble methane monooxygenase (sMMO) and related metalloenzymes. The BS HDFT (UB3LYP) method was employed to elucidate electronic and spin structures of the key intermediate 'Q' and to locate transition structures for hydroxylation reactions in the lowest-spin (LS) singlet and the highest-spin (HS) states of sMMO. The spin density populations and chemical indices obtained by the BS B3LYP calculations were found to be consistent with orbital interaction models for hydroxylation with MMO. However these indices in turn indicated significant spin contamination errors in the BS LS solution. The elimination of the errors with the AP procedure indeed reduced the barrier height for the recombination step of alkyl and hydroxyl radicals in the pure LS singlet state, leading to a rebound process. Then present computational results indicated that hydroxylation reactions proceed through the continuous diradical (diradicaloid) mechanism without discreet free radical fragments in the pure LS singlet state. The computational results are, respectively, compatible with local singlet (SD) and local triplet (TD) diradical mechanisms for hydroxylation in the LS and HS states; those were already applied to P450 successfully. Thus magnetic (exchange) coupling modes (LS and HS) in MMO, P450 and related metalloenzymes are directly related to local SD and TD mechanisms for hydroxylation, indicating the correspondence between the magnetic coupling mode and the radical reaction mechanism. These theoretical results enable us to examine recent BS hybrid DFT computational results for hydroxylation reactions with sMMO by several groups. Implications of the present theoretical and computational results are also discussed in relation to several experimental aspects of hydroxylation reactions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [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 have computed current–voltage character of natural DNA and its proton-transferred structure by scattering theory based on density functional theory. The current is not observed if the electron path contains only hydrogen bonding such as one base pair. The current becomes larger if the electron path contains π–π stacking molecule such as two base pairs. We also found that the conductivity of pseudo-ion pair (C+G-/G-C+), which is derived from proton-transfer reaction in CG/GC, differs from that of the original structure. On the other hand, the current changes dramatically if the electrode connects to guanine or cytosine, which can be explained by the difference of electron affinities. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

An approximate spin projection (AP) method is applied to a second derivative of energy (Hessian) of the broken-symmetry (BS) method in order to eliminate a spin contamination error. Stretching frequencies of F2 (X1 ∑g+) and singlet molecular oxygen (a1Δg) are calculated by using the spin-projected Hessian (AP Hessian). In case of F2 molecule, a potential curve of an unrestricted Hartree-Fock (UHF) method does not have an energy minimum but the stretching frequency can be calculated by the AP UHF method. The difference in the calculated stretching frequencies between a restricted HF (RHF) and the AP UHF methods is 650–700 cm-1, and indicates that a static correlation correction is a crucial to consider the frequency. On the other hand, we cannot find a significant difference in calculated frequencies of RHF and AP-BS HF in case of 1O2 molecule within the HF method. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

The nature of Cr&bond;Cr bond and effectiveness of broken-symmetry (BS) density functional theory (DFT) and hybrid DFT (HDFT) methods are examined in terms of electron correlation effects. Two types of correlation energies, i.e., static and dynamical correlation energies are estimated by relative energy curves and correlation energy curves. Energy decomposition analyses and natural orbital analyses reveal how these electron correlation effects affect the Cr&bond;Cr bonding. In addition, the effect of those effects to an energy gap between the lowest spin (LS) state (S = 0) and the highest spin (HS) state (S = 6) is also discussed with respect to calculations of effective exchange integrals (J). © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

In the case of the precious metal catalysts, the precious metal nanoparticles deposited on the several supports exhibit extremely high-catalytic activity for many catalytic reactions. The boundary region between the nanoparticles and the support is one of the active sites in these catalysts. Moreover, the core/shell-type bimetallic nanoparticles also show the high-catalytic activities for several catalytic reactions. In these systems, the electronic states of the surfaces in the clusters are modified by the heterojunction between the two different compositions. Therefore, we investigate the heterojunction effect in these model catalysts, such as precious metal core/shell clusters and Pd supported on single-wall carbon nanotube model cluster, using hybrid density functional theory. From the calculation results, we find that the charge transfer interactions and the variation of the ground spin states in the metal clusters are the characteristics induced by the heterojunction in these model systems. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [ABSTRACT FROM AUTHOR]

Chemical bonding and magnetic interaction of manganese dimer (Mn2) are still in controversy in experimental and theoretical studies. In this work, we examined various exchange correlation functionals of Kohn–Sham density functional theory (DFT), together with hybrid-DFT, and Hartree–Fock plus DFT approaches on description of Mn2. We found that, in contrast to pure DFT functionals, the experimentally reported features of chemical bonding and antiferromagnetic interactions of Mn2 can be reproduced by employing the Hartree–Fock exchange terms. Thus, it is recommended to employ HF plus DFT, or hybrid DFT with large HF portion approach for the case that the direct interactions among manganese atoms play a major role to determine the electronic structure, such as manganese clusters. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

Abstract: The one-dimensional intermolecular ferromagnetic interaction of a charge-transfer (CT) complex, (BDTA)[Ni(mnt)2] (BDTA: 1,3,2-benzodithiazolyl, mnt: maleonitriledithiolate), is studied by the Kohn–Sham hybrid density functional method, in order to understand the mechanism of the magnetism. All possible effective exchange integrals, J, between the spin pairs of the system indicate that a ferromagnetic interaction (J =172cm−1) exists along the b-axis. Magnetic susceptibility, simulated with the ab initio determined J values by the quantum Monte Carlo (QMC) method, is qualitatively consistent with that measured experimentally. [Copyright &y& Elsevier]

Our theoretical efforts toward molecule-based magnetic conductors and superconductors on the basis of ab initio Hamiltonians and effective model Hamiltonians are summarized in relation to recently developed DNA-based molecular materials. Guanine and adenine derivatives coupling with organic radicals (R) are investigated as possible π–R components. To elucidate electronic and magnetic properties of these species, effective exchange integrals (Jab) for magnetic clusters are calculated by ab initio hybrid density functional (HDFT) methods. The ab initio Jab values are numerically reproduced by using model Hamiltonians such as the t-J, Kondo, Anderson, and RKKY models. The theoretical possibilities of organic magnetic conductors are elucidated on the basis of these models for self-assembled DNA wires, sheets, and related materials. The use of these materials for nanoscale molecular electronic devices is also elucidated theoretically in relation to an important role of electron–electron repulsion effect for quantum electron transport, together with the electron-exchange interaction in the Kondo effect. The implications of the calculated results are finally discussed to obtain a unified picture of many p–d, π–d, and π–R molecule-based systems with strong electron correlations. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [ABSTRACT FROM AUTHOR]

Theoretical calculations of ab initio molecular orbital and density functional theory calculations were carried out in order to investigate magnetic interaction in one‐dimensional spin chains. Effective exchange integral (Jab) values are useful parameters in studying magnetic interaction in spin systems. Chemical indices also were evaluated to represent stability of broken symmetry spin. For one set of spin chains, alignment of hydrogen atoms as the most simple organic cluster was employed. Number of H atoms in employed models is two and 50 with interatomic distance 2.1 Å. In addition, copper bromide chains (CunBrm) were also studied. Simple molecules Br2‐Cu‐Br2‐Cu‐Br2 (without apical‐Br) and Br2‐CuBr‐Br2‐CuBr‐Br2 (with apical‐Br) were employed for simple model structure. Four parameters (r, a, b, and c) can fix the structure and study parameter dependency of Jab values. For experimental studies, Sugimoto and his coworkers have reported synthesis and experimental studies for complex of both CunBrm chains and TTF‐derivative donor. Magnetic behavior in only CunBrm chains was investigated in detail by several ab initio methods. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 [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]

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 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]

Abstract: The luminescent cyclometalated platinum(II) complex [Pt(HL)Cl] (1) (where HL = 2-phenyl-6-(1H-pyrazol-3-yl)-pyridine anion) and its deprotonated dimer complex [Pt2(L)2] (2) were theoretically studied via density functional theory (DFT) and time-dependent DFT (TDDFT) in conjunction with the polarizable continuum model (PCM) solvent effect to investigate their molecular geometries (at the S0, S1, and T1 states), electronic structures, and photophysical properties (absorption and phosphorescence spectra) and to elucidate the reason for the significantly higher phosphorescence quantum yield of 2 (59%) compared to that of the 1 (14%). [Copyright &y& Elsevier]

Abstract: Hybrid DFT (HDFT) calculations were carried out for model clusters of paranitrophenyl nitronyl nitroxide (p-NPNN) whose geometrical structures were based on its β-phase crystal structure. The effective exchange integrals between the neighboring p-NPNN molecules were calculated for the qualitative understanding of the pressure-induced transition form ferro- to antiferro-magnetic state. In the high pressure-induced condition, it could be presumed that the p-NPNN molecules in the crystal was gradually shifted along several lines or was rotated around appropriate axes. From these model calculations, it was found that the ferromagnetic exchange interaction between the neighboring dimers changed to antiferromagnetic one by the structure deformations. [Copyright &y& Elsevier]

The effective exchange integral (Jab) values between chromium ions in the aligned metal complexes; Cr2(pyphos)4 and Pt2Cr2(pyphos)4(CH3)4 were investigated theoretically. Full model calculations presuming chromium(II) that were usually found in the Cr2(O2CR)4L2 complexes could not reproduce the small experimental Jab values, suggesting another oxidation state of Cr. Several electronic states presuming Cr(III) were examined to explain the experimental Jab values using naked model. The Cr(III) dimer model which had π4δ2 triple bond showed small Jab value that could explain experimental Jab value of Cr2(pyphos)4. The calculated Jab value presuming π4δ2 triple bond of Cr(III) core also indicated appropriate Jab values which could explain the very small experimental Jab of Pt2Cr2(pyphos)4(CH3)4. Natural orbital and instability analysis were performed to investigate such small Jab value in Pt2Cr2(pyphos)4(CH3)4. The conjugation of Pt&z.sbnd;Cr&z.sbnd;Cr&z.sbnd;Pt core seemed to be effective in the decrease of Jab between the Cr ions. [Copyright &y& Elsevier]

Stable carbene:bis(9-(10-phenyl)anthryl)carbene and model carbenes of which have been investigated by ab initio MO and Crystal orbital calculations. By hybrid density-functional calculation (B3LYP/4-31G) the carbene has character as a triplet carbene than a triplet diradical. Based on calculations on this carbene, a polycarbene is contrived to investigate interactions of carbenes, which are stabilized by aromatic rings. They have interacted with each other in antiferromagnetic fashion by both B3LYP MO and Crystal orbital calculations. Their magnetic interactions have been varied as the conformational changes of the aromatic rings, which have been evaluated by the effective exchange integral J(AP) based on the Heisenberg Hamiltonian. To describe the behavior AO-approach has been introduced and has worked efficiently. [Copyright &y& Elsevier]

Abstract: The barrier height for automerization of cyclobutadiene (CBD) is examined by broken-symmetry (BS) HF, post-HF, and density functional methods. To remove the spin contamination error, we use an approximate spin projection (AP) method. Comparison between the results of BS methods and those of the localized UHF natural orbital (ULO) based Mukherjee’s state-specific MRCC (ULO-MkCC) method shows that spin-unrestricted Brueckner doubles including perturbative triple excitations (UBD(T)) with the AP correction performs well within a framework of single reference approaches. On the other hand, UDFT methods except for UBLYP and UM06-2X give negative barrier heights, which are not consistent with the both the experimental and MRCC results. [ABSTRACT FROM AUTHOR]

Abstract: We performed broken-symmetry (BS) B3LYP calculations on several model complexes of the active site of oxymyoglobin. The present study focuses on the effects of the orientations of His64 and His93 on the chemical bonding of the FeO2 core in the ground (S =0) state. The orientation of binding O2 molecule as well as axial His93 drastically changes during the optimization when using the BS method. In order to make this point clear, the spin contamination error involved in the BS singlet state is removed from both the energy and its gradient by an approximate spin-projection (AP) method. In most cases, the attempt to correct the geometry results in failure, indicating the Heisenberg model cannot be always applicable to this system. Judging from the difference in the structural parameters between the BS and corrected BS (AP) structure, the spin contamination error on the geometry of the FeO2 core is small. Based on these results, we suggest that the geometry optimization using a small model complex such as FeO2(Im)(Por) (Im, imidazole; Por, porphyrin) provides a local minima, in which π bond of Fe–O2 core is artificially stabilized by rotating the O2 molecule. [Copyright &y& Elsevier]

Abstract: Several hybrid density functional sets with a broken-symmetry (BS) method: UB3LYP, UB3PW91, UPBE0, UB(38HF)P86, and UBHandHLYP are examined on the geometry of the Cu2O2 core of the active site of oxyhemocyanin (oxyHc). In order to make a nature of each functional set clear, a spin contamination error involved in the BS singlet (S =0) state is removed from an energy gradient by an approximately spin-projected geometry optimization (AP-opt) method. After the AP-opt correction, both UB3LYP and UBHandHLYP give more accurate results for the distance between the center of mass of O–O and that of Cu–Cu that is often represented as a reaction coordinate R. In the viewpoint of structural and physicochemical properties, reliabilities of these functional sets are in the order of UBHandHLYP>UB3LYP≫UB3PW91>UB(38HF)P86>UPBE0. [Copyright &y& Elsevier]

An approximate spin projection (AP) method is applied to a second derivative of energy (Hessian) of the broken-symmetry (BS) method in order to eliminate a spin contamination error. Stretching frequencies of F2 (X1 ∑g+) and singlet molecular oxygen (a1Δg) are calculated by using the spin-projected Hessian (AP Hessian). In case of F2 molecule, a potential curve of an unrestricted Hartree-Fock (UHF) method does not have an energy minimum but the stretching frequency can be calculated by the AP UHF method. The difference in the calculated stretching frequencies between a restricted HF (RHF) and the AP UHF methods is 650–700 cm-1, and indicates that a static correlation correction is a crucial to consider the frequency. On the other hand, we cannot find a significant difference in calculated frequencies of RHF and AP-BS HF in case of 1O2 molecule within the HF method. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

Abstract: Hybrid density functional theory (HDFT) calculations have been carried out for clusters of p-NPNN extracted from the experimental crystal structures of the α-, β-, γ- and δ-phases in order to investigate the weak magnetic interactions between the organic radicals theoretically. From the systematic HDFT calculations for β- and γ-phase p-NPNN clusters, it was found that the magnetic long-range ferromagnetic and antiferromagnetic orderings would be presented in the β- and γ-phase p-NPNN crystals, respectively. [Copyright &y& Elsevier]

Abstract: The chemical species of bridge oxygen (WB) in active site of urease was examined in terms of a magnetic interaction between Ni(II) ions. The effective exchange integrals (J ab) values between Ni(II) ions were calculated by using three candidates of WB, i.e., H2O, OH− and O2−. The unrestricted hybrid density functional theory (UHDFT) calculations were carried out using the smallest models and realistic models. In the case of the smallest models, the experimental J ab value (−6.3cm−1) was a middle value of WB=H2O (−2.6cm−1) and WB=OH− (−11.4cm−1), while we could not obtain appropriate electronic structure using O2−-bridge model. The J ab became −16.7cm−1 by using more realistic OH−-bridge model, while one spin density on Ni2 ion was delocalized to Asp360 in case of H2O-bridge model. Natural orbital analyses revealed that the magnetic orbital through Lys-bridge delocalized to Asp360 and it seemed to associate with the spin transfer. From the results, OH−-bridge seemed to be the most possible candidate for the structure of active site in urease. [Copyright &y& Elsevier]

Abstract: The phase diagram of the metal–ligand (M–X) bond formation was formulated analytically. By using covalent bond parameter and ionic parameter, the chemical bonding and its dissociation nature of M–X bond were discussed. The electronic structure of cobalamin, which involves Co–C bond, was calculated by using UB3LYP method. Those calculations revealed the importance of corrin ring for the Co–C bond formation. The Co–C bond in the active site was covalent bond but potential minimum was close to bifurcation point between broken symmetry (BS) and spin adapted (SA). The covalent bond parameter and ionic parameter were estimated by using calculated effective exchange integrals (J), transfer integrals (t) and on-site Coulomb repulsion (U). By using those physical parameters, bond nature of Co–C in the cobalamin was discussed together with obtained phase diagram. [Copyright &y& Elsevier]