Four electronically low-lying states of CH_{2} 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 ^{
1}A_{1}) 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 CH_{2}
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 ^{
3}B_{1}), 1.690 (a ^{1}A_{1}),
0.669 (b ^{1}B_{1}), and 0.205 debye (c
^{1}A_{1}), respectively. With the most
accurate method in this study, the CASSCF-SOCI level with
the TZ3P(2f,2d)+2diff basis set, the energy separation
(T_{0} value) between the ground state (X ^{
3}B_{1}) and first excited state (a ^{
1}A_{1}) 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
T_{0} value for the second excited state (b ^{
1}B_{1}) 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 ^{
1}A_{1}) the T_{0} 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 ^{1}A_{1} state is determined to be
r_{e} = 1.0639 angstrom and theta_{e} =
171.62 degrees at the TCSCF-CISD level with the largest
basis set.