Application of the equations-of-motion method to the excited states of N2, CO, and C2H4

We have used the equations-of-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 Schrödinger's equation separately for the absolute energies and wavefunctions. We have found that by including both single particle-hole and two particle-hole components in the excitation operators we can predict the excitation frequencies of all the low-lying states of these three molecules to within about 10% 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.