Combined SCF and CI Method for the Calculation of Electronically Excited States of Molecules: Potential Curves for the Low-Lying States of Formaldehyde

Abstract

The success of various ab initio methods for the calculation of electronic spectra for polyatomic molecules is discussed with reference to the formaldehyde system. It is found that a limited CI calculation based on the SCF MO's of the ground-state wavefunction yields inaccurate potential curves for the excited states of H2CO and overestimates their transition energies from the ground state. It is concluded that the major cause for these deficiencies lies in the fact that the ground-state MO's do not represent a good starting point for the description of the excited states, and thus a second series of calculations is carried out using open-shell SCF techniques in an attempt to improve upon this situation. The resulting excited-state potential curves do show the proper angular behavior expected from experimental studies, but the calculated transition energies are too low because the correlation energy error in the SCF treatment is greater for the closed-shell ground state than for the open-shell excited species. To correct this shortcoming a limited CI calculation is carried out for each excited state based on the SCF MO's of its parent or main configuration. This combined SCF and CI treatment leads to excellent results for both transition energies and potential curves of ground and excited states and also produces relatively good dipole moments for both types of states.