A systematic study of the (X)over-tilde B-2(1), (A)over-bar (2)A(1), and (B)over-bar B-2(2) states of the neutral radical PH2

The three lowest-lying electronic states of PH2, (X) over tilde B-2(1), (A) over tilde (2) A(1), and (B) over tilde B-2(2), have been investigated sys tematically using ab initio electronic structure theory. The SCF, CASSCF, C ISD, CASSCF-SOCI, CCSD, and CCSD(T) levels of theory have been employed to determine total energies, equilibrium structures, and physical properties, including dipole moments, harmonic vibrational frequencies, and infrared in tensities. The predicted geometries and physical properties of the two lowe st states of PH2 are in good agreement with available experimental results. At the CCSD(T) level of theory with the correlation-consistent quadruple-x i basis set (cc-pVQZ), the (A) over tilde (2)A(1) State of PH2 has a large bond angle of 121.9 degrees and is predicted to lie 52.2 kcal/mol (2.26 eV, 18 300 cm(-1)) above the ground state. This is in excellent agreement with the experimental To values of 52.26 kcal/mol (2.266 eV, 18 276.59 cm(-1)) and 52.08 kcal/mol (2.258 eV, 18 215 cm(-1)). The second excited electronic state (B 2B2), not studied previously, was predicted to possess an unusual acute HPH angle of 29.1 degrees and a theoretical T-0 value of 71.3 kcal/m ol (3.09 eV, 24 900 cm(-1)) relative to the ground state.