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Instability in chemical bonds. II. Theoretical studies of exchange‐coupled open‐shell systems
Author(s) -
Yamaguchi K.,
Okumura M.,
Takada K.,
Yamanaka S.
Publication year - 1993
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560480848
Subject(s) - chemistry , methylene , open shell , binding energy , computational chemistry , chromium , wave function , spin (aerodynamics) , bond length , atomic physics , molecule , thermodynamics , physics , organic chemistry
Abstract The electronic and geometrical structures of exchange‐coupled open‐shell systems such as bis(methylene) methane and the chromium–methylene cation were investigated by the molecular orbital and its extended methods: RHF , UHF , UNO ‐ CAS , RMP , UMP , PUMP , APUMP , UCCSD ( T ), UNO ‐ CI and QCISD ( T ). The fully optimized geometries of these species by the energy gradients of UMP n solutions are consistent with the experimental values. The total energies of the lowest and highest spin states were calculated in order to elucidate the effective exchange integrals in the Heisenberg model for methylene and bis(methylene) methane. The spin projection is necessary for the UHF ‐based wavefunctions in the lowest spin state of the species. The binding energies for the chromium cation–methylene double bond were calculated by both spin‐unprojected and ‐projected methods. It was found that the binding energy between Cr + and 3 CH 2 by APUHF QCISD ( T ) is in accord with the experimental value. © 1993 John Wiley & Sons, Inc.