The 1 2A″, 1 2A′, and 2 2A′ electronic states of the H 2S + ion studied using multiconfiguration second-order perturbation theory

Abstract

Doublet electronic states of the H 2S + ion, within the C s as well as C 2 v symmetries, were studied using the CASSCF and CASPT2 methods in conjunction with an ANO basis. For the three lowest-lying C 2 v (1 2B 1 1 2A 1 and 1 2B 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 2B 2 be inappropriate since the calculated T 0 values for 1 2B 2 are about 1 eV smaller than the experimental T 0 value for the B state. The potential energy surfaces (PESs) of the three lowest-lying C s (1 2A″, 1 2A′, and 2 2A′) states were explored by locating and characterizing stationary points and by calculating minimum energy curves E( θ) ( θ = ∠HSH). In the 1 2A″ PES there is a minimum M0. In the 1 2A′ 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 2B 1, 1 2A 1, and 1 2B 2 C 2 v minima, respectively, while TS has a C s geometry. The CASSCF and CASPT2 geometries and CASSCF frequencies for M0 (1 2A″) and M1 (1 2A′) are in reasonable agreement with the experimental data for the X and A states of the H 2S + ion, respectively. The CASPT2 relative energy value of 2.38 eV for M1 (to M0) is in good agreement with the experimental T 0 value of 2.31 eV for the A state. We assign the A state to 1 2A′ (M1). The local minimum M2 in the 1 2A′ PES is not involved in any adiabatic excitation process of the H 2S + ion. In the 2 2A′ PES, a minimum M3 was located at a C 2 v 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 0 value of 4.31 eV for the B state, and we then assign the B state to 2 2A′ (M3).