Equations-of-motion method: Potential energy curves for N2, CO, and C2H4

We have applied the equations-of-motion method to various states of N2, CO, and ethylene at nuclear configurations slightly distorted from the ground equilibrium geometry. This approach attempts to calculate energy differences instead of absolute energies and is thus relatively insensitive to the accuracy of the assumed ground state wavefunction. By using the experimental behavior of the ground state on distortion, we can generate accurate potential energy curves for the excited states in the region of spectroscopic interest. These curves confirm the spectroscopic behavior of the ∑1u+ states of N2 and the 1Σ+ states of CO where valence and Rydberg states of the same symmetry interact. The results for the T and V states of ethylene agree with experiment and show that the V state is predominantly a highly correlated valence state. Oscillator strengths across an absorption band are also accurately determined in this method. We report the dependence of the transition moment on bond length for the X 1Σ+ → A 1Π transition in CO, which is in excellent agreement with experiment.