Four electronically lowest-lying ((X) over tilde(3)B(1), (a) over tilde(1)A(1), (b) over tilde(1)B(1), and (c) over tilde(1)A(1)) states of CH2 have been investigated systematically using ab initio electronic structure theory. Complete active space (GAS) self-consistent-field (SCF) second-order configuration interaction (SOCI) and state-averaged (SA) CASSCF-SOCI levels of theory have been employed. The CASSCF reference wave function was constructed by minimizing the total energy of a specified state, while the SACASSCF reference wave function was obtained by minimizing the equally weighted total energy of the four ((X) over tilde(3)B(1), (a) over tilde(1)A(1), (b) over tilde(1)B(1), and (c) over tilde(1)A(1)) states. The third excited state ((c) over tilde(1)A(1) or 2(1)A(1)) is of particular theoretical interest because it is represented by the second root of CASSCF and SOCI Hamiltonian matrices. Theoretical treatments of states not the lowest of their symmetry require special attention due to their tendency of variational collapse to the lower-lying state(s). For these four lowest-lying states total energies and physical properties including dipole moments, harmonic vibrational frequencies, and associated infrared (LR) intensities were determined and compared with the results from the configuration interaction with single and double excitations (CISD) method and available experimental values. The CASSCF-SOCI method should provide the most reliable energetics and physical properties in the present study owing to its fully variational nature in the molecular orbital (MO) and CI spaces for a given state. It is demonstrated that the SACASSCF-SOCI wave functions produce results which are quite consistent with those from the CASSCF-SOCI method. Thus significantly increased application of the SACASSCF-SOCI method to the excited states of a wide variety of molecular systems is expected. (C) 1997 Elsevier Science B.V.
