``The X 1A1, a 3B1, A 1B1, and B 1A1 Electronic States of PH2+,'' T. J. Van Huis, Y. Yamaguchi, C. D. Sherrill, and H. F. Schaefer, J. Phys. Chem. A 101, 6955-6963 (1997).

Four electronically low-lying states of PH2+ 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 1A1) 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 PH2+ have bent structures and are of C2v 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 re = 1.415 Å and thetae = 93.1 deg (X 1A1), re = 1.403 Å and thetae = 121.7 deg (a 3B1), re = 1.417 Å and thetae = 124.7 deg (A 1B1), and re = 1.411 Å and thetae = 159.3 deg (B 1A1). At this level of theory, the dipole moments of the ground and first three excited states of PH2+ are predicted to be 1.056 D (X 1A1), 0.653 D (a 3B1), 0.751 D (A 1B1), and 0.324 D (B 1A1), which are large enough to make these states susceptible to microwave spectroscopic analysis. The energy separations (T0 values) between the ground (X 1A1) 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 3B1 <- X 1A1), 45.82 kcal/mol (1.99 eV, 16 030 cm-1: A 1B1 <- X 1A1), and 85.05 kcal/mol (3.69 eV, 29 750 cm-1: B 1A1 <- X 1A1). After comparison of theoretical and experimental data for isovalent systems studied at the same level of theory, error bars for the B 1A1 <- X 1A1 splitting are estimated to be +/- 1.5 kcal/mol (+/- 525 cm-1).