Configuration Interaction Studies of Ground and Excited States of Polyatomic Molecules. I. The CI Formulation and Studies of Formaldehyde

A configuration interaction (CI) procedure which is designed to produce energies and wavefunctions from relatively large scale computations on ground and excited molecular states is presented based on an orthonormal set of molecular orbitals. The method of generating configurations is referenced to the particular state of interest and involves a two-pass procedure: First, the generation of configurations by single and double excitations from selected important configurations (parents), subject to a threshold criterion, followed by diagonalization of the energy to obtain an initial approximation to the wavefunctions of interest; second, the use of these wavefunctions as parents and a repeat of the generation and diagonalization steps to produce the final CI wavefunction. CI studies of the A21,3 (n→π*), 1,3A1 (π→π*, n→3p), 1B1 (σ→π*, π→3s), and B21 (n→3s) states of formaldehyde are reported for several large Gaussian basis sets of near atomic Hartree–Fock quality based on molecular orbitals determined from ground and excited state SCF treatments and from certain elementary virtual orbital transformations. Calculated Franck–Condon transition energies are in excellent agreement with experimental results for those transitions which have been observed; of particular interest is the occurrence of a n→3p transition on oxygen in the region of the spectrum atound 8 eV.