We have used the equationshyphen;ofhyphen;motion method to study various states of N2, CO, and ethylene. In this approach one attempts to calculate excitation energies directly as opposed to solving Schrouml;dinger's equation separately for the absolute energies and wavefunctions. We have found that by including both single particlehyphen;hole and two particlehyphen;hole components in the excitation operators we can predict the excitation frequencies of all the lowhyphen;lying states of these three molecules to within about 10percnt; of the observed values and the typical error is only half this. The calculated oscillator strengths are also in good agreement with experiment. The method is economical, requiring far less computation time than alternative procedures.
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