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Second‐order intermediate Hamiltonian method: Pilot applications to vertical excitations in π‐electron systems
Author(s) -
Zaitsevskii Andrei,
Malrieu JeanPaul
Publication year - 1995
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.560550206
Subject(s) - hamiltonian (control theory) , degenerate energy levels , atomic orbital , excitation , electron , physics , wave function , perturbation (astronomy) , atomic physics , quantum mechanics , chemistry , mathematics , mathematical optimization
Abstract Second‐order intermediate Hamiltonian quasi‐degenerate perturbation theory is applied to calculate π‐electron vertical excitation energies of ethylene and trans ‐butadiene. The choice of model spaces follows the physical intuition, including the excitations within the π‐shell and leaving the σ‐orbitals doubly occupied. Model spaces are optimized using the state‐average CASSCF procedure. The agreement between calculated and experimental transition energies is better than 0.1 eV. The importance of a decontracted treatment of the model‐space projections of wave functions is demonstrated. A simple model study of deviations from size‐consistency is reported and implications for calculations on larger molecules are briefly discussed. © 1995 John Wiley & Sons, Inc.