# A systematic approach to transition moment calculations

## Abstract

Extensive configuration interaction (CI) and multiconfiguration self-consistent-field (MCSCF) calculations on the A-X transitions of CH and CH+ are reported, which show how the transition dipole matrix element between two electronic states of a molecule can be determined accurately and systematically by a sequence of ab initio calculations. Two different computational approaches are investigated: one uses different orbitals for the two states involved and the other uses the same orbitals for both states. Both methods are shown to converge rapidly when the active orbital space is increased in such a way that all orbitals of similar importance are included at the same time. It is also shown that natural orbital occupation numbers from second-order CI calculations using a valence active orbital space provide an useful guide to the relative importance of natural orbitals; all orbitals of comparable occupation numbers must be included together. © 1988 American Institute of Physics.