An examination of the 2 1A1 states of formaldehyde and ketene including analytic configuration interaction energy first derivatives for singlet excited electronic states of the same symmetry as the ground state

In an accompanying paper by Lee, Allen, and Schaefer [J. Chem. Phys. 87, 7062 (1987)], an efficient formulation for the analytic evaluation of two-configuration self-consistent-field configuration interaction (TCSCF-CI) energy first derivatives is presented. In this paper the TCSCF-CI gradient method is adapted and applied to singlet excited electronic states of the same symmetry as the ground state. Since single-configuration self-consistent-field configuration interaction (SCF-CI) wave functions are unreliable for such electronic states due to the possibility of variational collapse, the TCSCF-CI method based on excited-state orbitals is the simplest means by which dynamical electron correlation can be incorporated effectively. Geometrical structures, excitation energies, and harmonic vibrational frequencies obtained with double-zeta plus polarization (DZP) and DZP+Rydberg (DZP+R) basis sets are reported for the 2 ¹A₁ states of formaldehyde and ketene. The 2 ¹A₁ state of H₂CO is found to have two distinct minima with C_2v symmetry: a (π→π*)¹ valence minimum with r_e(C-O)=1.564 Å and T₀=8.50 eV, and an (n→3p_y)¹ Rydberg minimum with r_e(C-O)=1.216 Å and T₀=7.93 eV. Some question exists on whether the (n→3p_y)¹ state occurs experimentally at 8.11 or 7.96 eV, but in either case the agreement between theory and experiment is good. The 2 ¹A₁ state of CH₂CO is found to have only one C_2v minimum with r_e(C-O) =1.129 Å and r_e(C-C)=1.403 Å. This state is predominantly Rydberg in character and can be designated (π→3p_x)¹, but evidence is presented which suggests a moderate amount of Rydberg-valence mixing involving the (π→π*)¹ ``state.'' The DZP+R CISD T₀=6.94 eV prediction confirms previous assignments of the experimental band at 6.78 eV to the 2 ¹A₁ state.