Browsing by Author "Yamamoto, Shigeyoshi"

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Choosing a proper complete active space in calculations for transition metal dimers: ground state of Mn(2) revisited
(2008)
Camacho, Cristopher
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Yamamoto, Shigeyoshi
;
Witek, Henryk A.
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應用化學系分子科學碩博班
;
Institute of Molecular science
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 angstrom, 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.
Intruder States in Multireference Perturbation Theory: The Ground State of Manganese Dimer
(2009-02-01)
Camacho, Cristopher
;
Witek, Henryk A.
;
Yamamoto, Shigeyoshi
;
應用化學系
;
應用化學系分子科學碩博班
A detailed analysis of a severe intruder state problem in the multistate multireference perturbation theory (MS-MRPT) calculations on the ground state of manganese dinner is presented. An enormous number of detected intruder states (> 5000) do not permit finding, even an approximate shape of the X(1) Sigma(+)(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 X(1) Sigma(+)(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) arc tested. None of these conventional techniques is able to fully avoid the quasidegeneracies. We believe that the MS-MRPT calculations on the three lowest A(g) states of manganese dimer constitute a perfect benchmark case for studying the behavior of MRPT in extreme Situations. (C) 2008 Wiley Periodicals, Inc.