The X̃ 3B1, ã 1A1, b̃ 1B1, and c̃ 1A1 Electronic States of CH2

Abstract

Four electronically low-lying states of CH2 have been investigated systematically by using ab initio electronic structure theory. Self-consistent-field (SCF), two-configuration self-consistent-field (TCSCF), complete active space self-consistent-field (CASSCF), configuration interaction with single and double excitations (CISD), and CASSCF second-order configuration interaction (SOCI) levels of theory were employed with nine different basis sets. Special emphasis is placed on the second and third excited states. The third excited state (c 1A1) is of particular theoretical interest because it is represented by the second root of TCSCF, CASSCF, TCSCF-CISD, and CASSCF-SOCI wave functions. Theoretical treatments of states not the lowest of their symmetry are traditionally treacherous. It is confirmed in this study that the four low-lying states of CH2 all have bent structures. For these four states total energies and physical properties including dipole moments, harmonic vibrational frequencies, and associat...