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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]

Electronic structures of the Compound I (CpdI) in P450 are investigated on the basis of spin coupling forms of iron-oxo (Fe(IV)&dbond;O) cores and radical ligand (•L) groups to generalize previous singlet and triplet diradical (TD) mechanisms for oxygenations of alkanes with Fe( IV)&dbond;O. Orbital interaction schemes for four lower-lying spin configurations of CpdI with H&bond;C bond of substrate are examined to elucidate how magnetic coupling modes correlate with radical reaction pathways for hydroxylation reactions on the basis of the broken symmetry (BS) molecular orbital (MO) model. The configuration correlation diagrams for the four configurations model are depicted on the basis of the isoelectronic analogy among O, O2, and Fe( IV)&dbond;O, in addition to Coulomb exchange energy on the iron site, which determines its local spin configuration. Important role of ligand spin (•L) of CpdI for regulation of hydroxylation mechanisms is clarified with the aid of the spin coupling forms. Transition states for one quartet and three doublet configurations under the BS MO approximation are examined on the basis of potential curve crossings along reaction pathways. The four transition structures and corresponding radical intermediates for methane and trimethyl methane with CpI are located by the BS hybrid Kohn–Sham density functional theory (DFT) (B3LYP) method to confirm the orbital interaction schemes. Spin density populations obtained by the BS B3LYP calculations are found to be consistent with the theoretical predictions based on the four configurations model. The configuration and state correlation diagrams by BS B3LYP before and after spin projection are also consistent with the BS MO interaction schemes, which provide local SD and TD mechanisms of hydroxylation with CpdI. The present BS MO-theoretical framework is useful for systematic understanding of a lot of recent BS hybrid DFT computational results for hydroxylation reactions with CpdI and configuration correlation diagrams reported by several groups. Implications of the present theoretical and computational results are discussed in relation to several experimental characteristics of hydroxylation reactions with iron-oxo species and P450. © 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]

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We have investigated the reaction pathways for the primary hydroxylation reaction of trimethylmethane by a high-valent Fe(IV)&dbond;O porphyrin π-cation radical species known as compound I at the B3LYP/CEP-31G level. The isoelectronic analogy of the Fe(IV)&dbond;O core of compound I to a molecular oxygen (O2) has been successfully used to clarify the important roles of the singlet excited state of the Fe(IV)&dbond;O core in the alkane hydroxylation, which has hitherto been neglected. The reaction is initiated by the rate-determining hydrogen-atom abstraction from the substrate to give a discrete radical intermediate complex, in accordance with the conventional radical rebound mechanism. Similar to the chemistry of O2, however, one of the singlet excited states, i.e., the diradical component of the 1Δ state of the Fe(IV)&dbond;O core intercepts the triplet ground state (the 3Σ state) in the region of the transition state for the hydrogen abstraction. Our findings strongly indicate that the exchange polarization or intersystem crossing for the nonradiative transition to the locally singlet state is highly important to enhance the reactivity of compound I. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [ABSTRACT FROM AUTHOR]

A first principle investigation has been carried out for intermediate states of the catalytic cycle of a cytochrome P450. To elucidate the whole catalytic cycle of P450, the electronic and geometrical structures are investigated not only at each ground state but also at low-lying energy levels. Using the natural orbital analysis, the nature of chemical bonds and magnetic interactions are investigated. The ground state of the Compound 1 (cpd1) is calculated to be a doublet state, which is generated by the antiferromagnetic coupling between a triplet Fe(IV)&dbond;O moiety and a doublet ligand radical. We found that an excited doublet state of the cpd1 is composed of a singlet Fe(IV)&dbond;O and a doublet ligand radical. This excited state lies 20.8 kcal mol-1 above the ground spin state, which is a non-negligible energy level as compared with the activation energy barrier of ΔE# = 26.6 kcal mol-1. The reaction path of the ground state of cpd1 is investigated on the basis of the model reaction: 3O(3p) + CH4. The computational results suggest that the reactions of P450 at the ground and excited states proceed through abstraction (3O-model) and insertion (1O-model) mechanisms, respectively. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [ABSTRACT FROM AUTHOR]

Ligand coordination effects on the magnetic interaction and the reversible dioxygen binding process for the models of hemocyanin have been investigated with hybrid density functional theory. Hemocyanin is an oxygen transport protein found in mollusks and arthropods. The active site of hemocyanin contains a pair of copper atoms, each of which is coordinated by three histidine residues. We utilized NH3 (a), methylimidazole (b), and histidine (c) as the ligands of the models of hemocyanin, which coordinate to the binuclear copper core in the active site of oxygenated (1z) (z = a–c) and deoxygenated hemocyanin (2z). The difference of the ligand coordination in the magnetic couplings for the model of hemocyanin 1z complexed with peroxide (1z-O2-2) shows that the substitution from histidine to methylimidazole group is not serious, but that the substitution from imidazole to NH3 is influential. In the dioxygen binding process, the ligands influence the binding energy and the shapes of potential energy surfaces of the models of hemocyanin 1z and 2z with dioxygen due to the strength of the mixing between the ground state and the charge transfer one. The active dxy ± dxy orbitals of the copper core of 1b and 1c strongly interact with π*h and π*v orbitals of the dioxygen than those of 1a, leading to the larger binding energy of the model 1b (1c) with dioxygen. The orbital energies of the active orbitals for the synthetic model, [Cu(HB(3,5-iPr2-Pz)3)]2 (HB(3,5-iPr2-Pz)3 = hydrotris{3,5-diisopropyl-pyrazolyl}borate), imply that it irreversibly binds dioxygen because HB(3,5-iPr2-Pz)3 groups show stronger electron-release character and raise the orbital energies nearer that to π* orbitals of O2. The effective bond-order, localized electron density, and information entropy for 1z with dioxygen were calculated in terms of occupation numbers of the natural orbitals of the broken-symmetry UB2LYP solution in order to elucidate the nature of the Cu2O2 bonding. These chemical indices indicate that the copper–oxygen bonds in Hc exhibit an intermediate covalent-bonding character (half bonding), which is the origin of an active control of oxygen transport: addition and dissociation of O2 in hemocyanin by the conformational changes. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

The radical harvesting state of a ribonucleotide reductase R2 subunit (RNR-R2) is theoretically investigated at the broken-symmetry DFT level. Recently, a high-resolution X-ray crystallography revealed a precise location of the active site structure of RNR-R2 (Class 1b) from C. ammoniagenes, and found a linking water near the tyrosine (Tyr) radical site. In this study, the magnetic interaction between the Tyr radical and the diiron core JTyr&bond;Fe is investigated for the first time. Used theoretical model for the active site is composed of diiron core, the Tyr residue, and surrounding side-chain groups. After the geometrical structure optimization of the core structures, the magnetic interaction between the iron centers was calculated to be JFe&bond;Fe = -77.82 cm-1, which was comparable to the experimental result of JFe&bond;Fe = -84 cm-1. The natural orbital analysis clearly showed that the magnetic interaction between the two iron centers mainly interact through the p orbital of the bridging μ-oxo. It was also calculated that the JTyr&bond;Fe interaction is negligibly weak, and the Tyr radical is almost in an isolated or a free radical state. These results suggest that the linking water may be used for (1) a confinement of the reactive Tyr radical in the hydrogen bonds or (2) a hydrogen-transfer coupled with electron transfer at the radical formation step of the Tyr. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

Genetic algorithms (GAs) were applied to the search for ground states of randomly generated spin clusters. Two types of hybrid GAs and one type of pure GA modified by crossover operation were developed especially for the search for the ground state on Ising spin clusters. In the hybrid GAs, we used the 1-neighborhood search and the hill-climbing search for local searches. For modification of GAs, we also proposed a new crossover operator, &xmacr; crossover, which turns all spins upside down. The fitness function (F(x)) was defined in terms of the energies obtained by the Ising Hamiltonian. For the Ising spin clusters, the effective magnetic interaction (Jab) values were calculated with the unrestricted Hartree–Fock method. These GAs worked well on the searches for the ground states, because the improved GAs avoided being trapped in the local minima frequently and picked up the global minima more effectively than the standard GAs. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [ABSTRACT FROM AUTHOR]

A quantum spin-correction scheme based on spin-correlation functions is proposed for multiple bond-breaking processes described by the unrestricted single determinant (USD) approaches, such as unrestricted Hartree–Fock (UHF) and unrestricted Kohn–Sham (UKS) density functional theory (DFT). It is known that natural orbitals (NOs) and occupation numbers (ONs) contain full information of USD solutions. We introduce a bond-operator representation of UHF and UKS wave functions to exploit information about those NOs, enabling us not only to facilitate the calculus, but also to understand the essential feature of the USD and those quantum spin-corrected solutions. We found that usual spin-projected solutions do not work for multiple bonds in the Heisenberg model mapping scheme. We propose quantum spin-correction schemes to obtain the spin-symmetry allowed solutions from the spin-unrestricted KS-DFT solutions. We examine these schemes for a chromium dimer, which is a typical example exhibiting multiple bonds. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [ABSTRACT FROM AUTHOR]

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]

A strategy is proposed to determine parameters in model Hamiltonians from the difference of the density matrices calculated by ab initio methods. As an example, the parameters of the two‐site Hubbard model are determined. The parameters t and U are explicitly expressed in terms of the energy difference and the difference of the expectation values of the spin operator 〈S2〉 between two unrestricted Hartree–Fock (UHF) solutions. By using this relation and the exact energy formulas of the model Hamiltonian the energies of the ground and excited states of the ab initio Hamiltonian are estimated. This energy estimation scheme is applied to H2 and Li2 molecules and the results are in reasonable agreement with those of the full configuration‐interaction method. The errors of our estimation are also discussed and evaluated by using a modified Hubbard model. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [ABSTRACT FROM AUTHOR]

In a previous paper we found that the cofacial intermolecular π–π orbital interaction in stacking dimers significantly changes the longitudinal second hyperpolarizability (γ) of the isolated monomer. On the basis of this result, we investigate the longitudinal γ values of π‐conjugated main chains (CnHn+2, 6 ≤ n ≤ 16) interacting in both‐end regions with two small‐size cationic perturbing π‐conjugated molecules, that is, allyl cations (C3H+5). These interacting model systems exhibit remarkable enhancement of γ values as compared with those of isolated main chains in the whole chain‐length region. The γ density analysis reveals that this enhancement is described by the virtual charge transfer between both‐end perturbing molecules via the main chain. The analysis of orbital correlation diagram between the perturbing molecules and main chain molecule also clarifies that such feature of γ density distribution originates in the “weak intermolecular antibondinglike coupling” between the (lowest unoccupied molecular orbital [LUMO], LUMO+1) of cationic perturbing molecules and (highest unoccupied molecular orbital [HOMO], HOMO−1) of main chain molecule. The current result suggests the possibility of novel nano‐size control of nonlinear optical (NLO) properties by adjusting the intermolecular orbital interactions between the main molecule and perturbing molecules. A possible control scheme of longitudinal γ for novel intermolecular interacting NLO systems using modified DNA wires is also proposed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [ABSTRACT FROM AUTHOR]

We investigate the quantum dynamics of a dissipative molecular system interacting with a single-mode coherent field (with an initial average photon number 8) using the Monte Carlo wave function (MCWF) method. As quantum dissipation, we consider cavity relaxation (the dissipation of an internal single mode to outer mode) and molecular population relaxation and molecular coherent (phase) relaxation caused by nuclear vibration. In this study, we elucidate the effects of these dissipations on the unique quantum dynamic behaviors of this coupled system, e.g., collapse–revival behavior of Rabi oscillations, using the diagonal and off-diagonal molecular density matrices. It is found that although slight cavity relaxation suppresses the revival of molecular population similarly to other relaxation effects with larger decay rates the cavity relaxation hardly suppresses the increase in the off-diagonal molecular density matrix (representing the degree of coherency between the ground and excited states) in the quiescent region, the feature of which is related to the generation of the Schrödinger cat state, in contrast to molecular phase and population relaxations. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [ABSTRACT FROM AUTHOR]

Stabilization effects of nonsynchronous transition structures (TSs) of the Cope rearrangements of phenyl-substituted 1,5-hexadienes were examined by the hybrid density functional theory (DFT) and UHF methods. Natural orbital analyses of the hybrid UDFT and UHF solutions were performed to analyze the instability of molecular orbitals at nonsynchronous TSs. It was found that the chemical characteristics at the TSs are made explicit in the behavior of the information entropy and related chemical indices, which are defined by the occupation numbers of natural orbitals. The computational results are discussed in connection with the utility of chemical indices and the applicability of the hybrid DFT methods for pericyclic reactions of large systems. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [ABSTRACT FROM AUTHOR]

The two-exciton dynamics in a dendritic molecular aggregate composed of monomers coupled with each other by the dipole–dipole interaction is performed to investigate the effect of two-exciton states on the exciton migration using the master equation approach. Although the spatial unique feature of exciton migration from the periphery to the core in the one-exciton model is also observed in the two-exciton model, about 36% of such exciton migration is found to be carried out by the relaxation among two-exciton states in the present model. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the scope and applicability of Mukherjee’s state-specific multireference coupled cluster singles and doubles (Mk-MRCCSD) method on the magnetic interactions in dichloro-, oxo-, and peroxo-bridged binuclear Cu(II) complexes. Several reference orbitals including broken-symmetry natural orbitals (BSNOs) are examined for the Mk-MRCCSD computations. We compare the effective exchange integral (J) values calculated by Mk-MRCCSD with those calculated by conventional complete active space self-consistent field (CASSCF) and MRMP2. The BS methods with approximate spin-projection (AP) method are also performed for comparison. It is found that the Mk-MRCCSD method can be applicable to the prediction of exchange couplings in binuclear copper(II) containing systems. [Copyright &y& Elsevier]

Ab initio MO and hybrid-DFT calculations have been carried out by using model clusters; a [(NH3)2]+nn for pure organic conductors and b [H2NO&z.sbnd;(NH3)2]+nn for pure organomagnetic conductors. The results have shown that these models are reliable for investigation of intrinsic electronic states of magnetic/conductive organic compounds in detail by quantum chemical methods. [Copyright &y& Elsevier]