``The X 3B1, a 1A1, b 1B1, and c 1A1 Electronic States of CH2,'' Y. Yamaguchi, C. D. Sherrill and H. F. Schaefer, James L. Kinsey Issue, J. Phys. Chem. 100, 7911 (1996).

Four electronically low-lying states of CH2 have been investigated systematically 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 associated infrared (IR) intensities were determined and compared with available experimental values. At the CISD level with the largest basis set, the triple zeta plus triple polarizations with two higher angular momentum and two diffuse functions basis set [TZ3P(2f,2d)+2diff], the dipole moments are determined to be 0.600 (X 3B1), 1.690 (a 1A1), 0.669 (b 1B1), and 0.205 debye (c 1A1), respectively. With the most accurate method in this study, the CASSCF-SOCI level with the TZ3P(2f,2d)+2diff basis set, the energy separation (T0 value) between the ground state (X 3B1) and first excited state (a 1A1) is predicted to be 9.025 kcal/mol (0.3914 eV, 3157 cm-1) which is in excellent agreement with an experimentally derived value of 8.998 kcal/mol (0.3902 eV, 3147 cm-1) by Jensen and Bunker [J. Chem. Phys. 1988, 89, 1327]. The T0 value for the second excited state (b 1B1) is determined to be 33.224 kcal/mol (1.4407 eV, 11620 cm-1) which is in good agreement with an experimental estimate of 32.546 kcal/mol (1.4113 eV, 11383 cm-1) by Jensen and Bunker above and by Alijah and Duxbury [Mol. Phys. 1990, 70, 605]. For the third excited state (c 1A1) the T0 value is predicted to be 59.079 kcal/mol (2.5619 eV, 20663 cm-1) with estimated error bars of +/- 1.50 kcal mol-1 or +/- 525 cm-1. The equilibrium geometry of this c 1A1 state is determined to be re = 1.0639 angstrom and thetae = 171.62 degrees at the TCSCF-CISD level with the largest basis set.