(X)over-tilde(1)A(1), (a)over-tilde(3)B(1), (A)over-tilde(1)B(1), and (B)over-tilde(1)A(1) electronic states of PH2+

Four electronically low-lying states of PH2+ have been investigated using several different ab initio methods and multiple basis sets. This systematic study of both method and basis set provides reliable benchmarking for estimation when high levels of theory are not attainable. 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) over tilde (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 PH2+ all have bent structures and are of C-2 nu symmetry. Also determined in this study for these four electronic states were relative energies and physical properties including dipole moments and harmonic vibrational frequencies with their 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 Angstrom and theta(e)=93.1 degrees ((X) over tilde (1)A(1)), r(e)=1.403 Angstrom and theta(e)=121.7 degrees ((a) over tilde B-3(1)), r(e)=1.417 Angstrom and theta(e)=124.7 degrees ((A) over tilde B-1(1)), and r(e)=1.411 Angstrom and theta(e) = 159.3 degrees ((B) over tilde (1)A(1)). At this level of theory, the dipole moments of the ground and first three excited states of PH: are predicted to be 1.056 D ((X) over tilde (1)A(1)), 0.653 D ((a) over tilde B-3(1)), 0.751 D ((A) over tilde B-1(1)), and 0.324 D ((B) over tilde (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) over tilde (1)A(1)) and three excited states predicted 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) over tilde B-3(1) <-- (X) over tilde (1)A(1)), 45.82 kcal/mol (1.99 eV, 16 030 cm(-1): (A) over tilde B-1(1)<--(X) over tilde (1)A(1)), and 85.05 kcal/mol (3.69 eV, 29 750 cm(-1); (B) over tilde (1)A(1)<--(X) over tilde (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) over tilde (1)A(1)<--(X) over tilde (1)A(1) splitting are estimated to be +/-1.5 kcal/mol (+/-525 cm(-1)). Adiabatic and vertical ionization potentials of PH2 are also presented.