Shigeyoshi Yamamoto

A study of the ground state of manganese dimer using quasidegenerate perturbation theory

We study the electronic structure of the ground state of the manganese dimer using the state-averaged complete active space self-consistent field method, followed by second-order quasidegenerate perturbation theory. Overall potential energy curves are calculated for the 1Sigmag+, 11Sigmau+, and 11Piu states, which are candidates for the ground state. Of these states, the 1Sigmag+ state has the lowest energy and we therefore identify it as the ground state. We find values of 3.29 A, 0.14 eV, and 53.46 cm(-1) for the bond length, dissociation energy, and vibrational frequency, in good agreement with the observed values of 3.4 A, 0.1 eV, and 68.1 cm(-1) in rare-gas matrices. These values show that the manganese dimer is a van der Waals molecule with antiferromagnetic coupling.

Intruder states in multireference perturbation theory: The ground state of manganese dimer

A detailed analysis of a severe intruder state problem in the multistate multireference perturbation theory (MS‐MRPT) calculations on the ground state of manganese dimer is presented. An enormous number of detected intruder states (> 5000) do not permit finding even an approximate shape of the X1Σ  g+ potential energy curve. The intruder states are explicitly demonstrated to originate from quasidegeneracies in the zeroth‐order Hamiltonian spectrum. The electronic configurations responsible for appearance of the quasidegeneracies are identified as single and double excitations from the active orbitals to the external orbitals. It is shown that the quasidegeneracy problem can be completely eliminated using shift techniques despite of its severity. The resultant curves are smooth and continuous. Unfortunately, strong dependence of the spectroscopic parameters of the X1Σ  g+ state on the shift parameter is observed. This finding rises serious controversies regarding validity of employing shift techniques for solving the intruder state problem in MS‐MRPT. Various alternative approaches of removing intruder states (e.g., modification of the basis set or changing the active space) are tested. None of these conventional techniques is able to fully avoid the quasidegeneracies. We believe that the MS‐MRPT calculations on the three lowest Ag states of manganese dimer constitute a perfect benchmark case for studying the behavior of MRPT in extreme situations.

CASSCF calculation on dioxygen heme complex with extended basis set

Abstract Ab initio CASSCF calculations on a model oxyhemoglobin complex are reported. An extended basis set is employed in order to obtain a reliable result. Electronic properties, including the charge distribution and the effects of electronic configuration mixing, are investigated for a realistic model compound. The oxygen moiety in the FeO 2 bond is found to be electrically almost neutral.

Ab initio RHF and CASSCF studies on Fe–O bond in high‐valent iron‐oxo‐porphyrins

The electronic structure of the Fe–O bond in highly oxidized iron porphyrins was elucidated by ab initio RHF (restricted Hartree–Fock) and CASSCF (complete active space SCF) calculations on a neutral complex FeP(py)O (P=porphine, py=pyridine), which is a model of peroxidase compound II. Accounting for the correlation effects is essential for the description of the Fe–O bond. Equilibrium distance and stretching frequency of Fe–O were calculated from the potential energy curves obtained by CASSCF. Furthermore, Mossbauer spectrum parameters (quadrupole splitting, isomer shift, asymmetry parameter and direction of the principal axis of the electric field gradient tensor) and spin density were evaluated. They are in good agreement with experiments on the whole. The oxidation number of iron in peroxidase compound II was assigned to IV through an analysis of the CASSCF wave function.

Choosing a proper complete active space in calculations for transition metal dimers: ground state of Mn2 revisited

The potential energy curve of the ground state of Mn(2) has been studied using a systematic sequence of complete active spaces. Deficiencies of the routinely used active space, built from atomic 4s and 3d orbitals, has been identified and discussed. It is shown that an additional sigma(g) orbital, originating from the atomic virtual 4p(z) orbitals, is essential for a proper description of static correlation in the (1)Sigma(g)(+) state of Mn(2). The calculated spectroscopic parameters of the (1)Sigma(g)(+) state agree well with available experimental data. The calculated equilibrium bond lengths are located between 3.24 and 3.50 A, the harmonic vibrational frequencies, between 44 and 72 cm(-1), and the dissociation energies, between 0.05 and 0.09 eV. An urgent need for an accurate gas-phase experimental study of spectroscopic constants of Mn(2) is highlighted.

CASSCF study on the FeO2 bond in a dioxygen heme complex

Abstract Ab initio complete active space self-consistent field (CASSCF) calculations have been performed to elucidate the electronic structure of the FeO 2 bond in an oxyheme model compound. The main configuration in the CASSCF ground-state wavefunction is 1 A′ (de) 6 (π g a ) 2 , but its weight is small (0.641). Two-electron excitations, (d π ) 2 →(π g b ) 2 and (π g a )(d π )→(π g b )(d σ ), mix into the dominant configurations.

Ab initio MO study on potential energy surfaces for twisting around C7C8 and C4–C7 bonds of coumaric acid

Abstract We have calculated potential energy surfaces (PESs) for the twisting around C7C8 and C4–C7 bonds of anionic form of coumaric acid with and without hydrogen bonds between its phenorate part and hydroxyl groups nearby, modeling the active site of Photoactive Yellow Protein. The calculation is made by means of the state-average complete active space self-consistent field theory. We revealed that the PESs of the ground and first excited states as a function of twisting angle around C7C8 are of strongly crossing type, and the crossing tendency becomes more prominent in the presence of the hydrogen bonds. We found that the PESs of the ground and first excited states as a function of twisting angle around C4–C7 bond are also of crossing type, representing double bond character.

An information-entropic study of correlated densities of the water molecule

The Shannon entropy of the water molecule was calculated at different correlation levels including full configuration interaction (CI) for the D95 basis set. The results show that an analysis of both the position and momentum space entropy yields insights into the characteristics of different correlated methods from the density perspective and provides an alternative way of interpreting the wave function. Various changes in the electronic densities intrinsic to these correlation methods are also related to concepts within the information entropy framework.

A theoretical study of the bent form of CuO2

Abstract The bent form of the CuO 2 complex is investigated using ab initio multi-reference singly and doubly excited CI calculations. The equilibrium geometry and vibrational frequencies are determined. The ground state is characterized by charge transfer from the Cu 4s orbital to the in-plane antibonding π orbital of dioxygen. The complex has a large dipole moment of 7.3 D with the direction Cu + O 2 − . The calculated bond length and stretching frequency of the dioxygen moiety in CuO 2 are 1.342 A and 1161 cm −1 , respectively. These values are very close to the corresponding values for the free superoxide, O 2 − . The calculated stretching frequencies of the complex are in good agreement with experiment.

A computational study on the stability of the protonated Schiff base of retinal in rhodopsin

Abstract We investigated the effect of amino acids in rhodopsin on the protonation state of the Schiff base (SB) retinal. We constructed a model system consisting of SB retinal, Glu113 (counterion), and eight residues. For this model, we considered two states of the SB retinal, namely, the protonated/deprotonated state. We then performed ab initio MO calculations at the RHF/6-31g* level. As a result, the protonated state was stabler than the deprotonated state. Interestingly, we observed an additive rule for the contribution to the stabilization energy due to each amino acid. Above all, it turned out that Ser186 and Cys187 play a significant role in the stability.