The 12A", 12A′ and 22A′ electronic states of the H2S+ ion studied using multiconfiguration second-order perturbation theory

Doublet electronic states of the H2S+ ion, within the C-s as well as C-2v symmetries, were studied using the CASSCF and CASPT2 methods in conjunction with an ANO basis. For the three lowest-lying C-2v (1(2)B(1) 1(2)A(1) and 1(2)B(2)) states, the CAS (CASSCF and CASPT2) calculation results (geometries, frequencies, and adiabatic excitation energies (T-0)) are similar to those of the previous MRD-CI and MRCI calculations. We consider that the previous assignment of the B state to 1(2)B(2) be inappropriate since the calculated T-o values for 1(2)B(2) are about 1 eV smaller than the experimental T-o value for the B state. The potential energy surfaces (PESs) of the three lowest-lying C-s (1(2)A", 1(2)A', and 2(2)A') states were explored by locating and characterizing stationary points and by calculating minimum energy curves E(theta) (theta = angle HSH). In the 1(2)A '' PES there is a minimum M0. In the 1(2)A' PES, there are the global minimum M1, a local minimum M2, and a first-order saddle point TS connecting M1 and M2. M0, M1, and M2 are identified with the 1(2)B(1), 1(2)A(1), and 1(2)B(2) C-2v minima, respectively, while TS has a C-s geometry. The CASSCF and CASPT2 geometries and CASSCF frequencies for M0 (1(2)A '') and M1 (1(2)A') are in reasonable agreement with the experimental data for the X and A states of the H2S+ ion, respectively. The CASPT2 relative energy value of 2.38 eV for M1 (to M0) is in good agreement with the experimental T-o value of 2.31 eV for the A state. We assign the A state to 1(2)A' (M1). The local minimum M2 in the 1(2)A' PES is not involved in any adiabatic excitation process of the H2S+ ion. In the 2(2)A' PES, a minimum M3 was located at a C-2v geometry at the CASPT2 level. The CASPT2 relative energy value of 4.19 eV for M3 (to M0) is quite close to the experimental T-o value of 4.31 eV for the B state, and we then assign the B state to 2(2)A' (M3). (C) 2005 Elsevier B.V. All rights reserved.