Four electronically low-lying states of
PH_{2}^{+} have been investigated using
several different *ab initio* methods and multiple
basis sets. 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 eight different basis sets of
triple-zeta quality. Being the second root of the TCSCF,
CASSCF, TCSCF-CISD, and CASSCF-SOCI wave functions, the
third excited state (B ^{1}A_{1}) is of
particular theoretical interest, for theoretical treatments
of states not the lowest of their symmetry are
traditionally very difficult. It is confirmed in this study
that the four low-lying states of
PH_{2}^{+} have bent structures and are of
C_{2v} symmetry. Also determined in this study for
these four electronic states were relative energies and
physical properties including dipole moments, harmonic
vibrational frequencies, and associated infrared (IR)
intensities. These properties were compared with
experimental values when possible. At the CISD level with
the largest basis set (triple-zeta plus triple
polarizations with two higher angular momentum and two
diffuse functions [TZ3P(2f,2d)+2diff]), the equilibrium
geometries of the four states are predicted to be
r_{e} = 1.415 Å and theta_{e} = 93.1
deg (X ^{1}A_{1}), r_{e} = 1.403
Å and theta_{e} = 121.7 deg (a ^{
3}B_{1}), r_{e} = 1.417 Å and
theta_{e} = 124.7 deg (A ^{
1}B_{1}), and r_{e} = 1.411 Å
and theta_{e} = 159.3 deg (B ^{
1}A_{1}). At this level of theory, the dipole
moments of the ground and first three excited states of
PH_{2}^{+} are predicted to be 1.056 D (X
^{1}A_{1}), 0.653 D (a ^{
3}B_{1}), 0.751 D (A ^{
1}B_{1}), and 0.324 D (B ^{
1}A_{1}), which are large enough to make these
states susceptible to microwave spectroscopic analysis. The
energy separations (T_{0} values) between the
ground (X ^{1}A_{1}) and three excited
states at the CASSCF-SOCI level with the TZ3P(2f,2d)+2diff
basis set are 17.74 kcal/mol (0.77 eV, 6200
cm^{-1}: a ^{3}B_{1} <- X ^{
1}A_{1}), 45.82 kcal/mol (1.99 eV, 16 030
cm^{-1}: A ^{1}B_{1} <- X ^{
1}A_{1}), and 85.05 kcal/mol (3.69 eV, 29 750
cm^{-1}: B ^{1}A_{1} <- X ^{
1}A_{1}). After comparison of theoretical and
experimental data for isovalent systems studied at the same
level of theory, error bars for the B ^{
1}A_{1} <- X ^{1}A_{1}
splitting are estimated to be +/- 1.5 kcal/mol (+/- 525
cm^{-1}).