Theoretical Calculations. BeS is a closed – shell ionic species with an experimentally verified X 1Σ+ ground state. The unpaired electron of BeS- resides in an orbital that is primarily of Be 2s – S 3pz antibonding character, giving rise to a X 2Σ+ ground state. Electronic structure calculations were performed on both the anion and neutral beryllium sulfide molecules. These calculations were done to assist in the assignment of the spectra, and to evaluate the suitability of the chosen electronic structure methods. For both beryllium and sulfur, the aug – cc – pwCVQZ basis sets were employed. In our studies4 of BeO/BeO- we found that it was advantageous to use density functional theory B3LYP wavefunctions as the reference set for the higher order calculations. Hence, the same approach was taken for BeS/BeS-. All calculations were performed using the MOLPRO suite of programs.37 Potential energy curves (PECs) were calculated pointwise. One series of calculations used the partially spin adapted coupled cluster method including single and double excitations and perturbative corrections for triple excitations (RCCSD(T)). The second method employed was a complete active space self-consistent field calculation, followed by multireference configuration interaction (CASSCF+MRCI+Q, denoted as MRCID in the following text) levels of theory. The Davidson correction38 was applied in order to partially compensate for the size inconsistency of the MRCI calculations. All electrons were included in the correlation treatment of the RCCSD(T) calculations. The active space in the CASSCF calculations consisted of the 2s and 2p orbitals of Be and the 3s and 3p orbitals of S. The Be 1s and S 1s, 2s, and 2p orbitals were constrained to be doubly occupied. Their wavefunctions were optimized in the CASSCF procedure to aid convergence. For both the anion and neutral BeS species, all electrons were included in the correlation treatment of the subsequent MRCI calculation, in an attempt to recover the core – valence correlation energy. The potential energy curves of the RCCSD(T) calculations are shown in Figure 3.1. Overall, the MRCID results were of comparable quality, with the exception that the error for the electron binding energy (eBE) was larger. The bound ro – vibrational levels of the resulting PECs were calculated using the LEVEL 8.0 program.39 The lowest vibrational energy levels were least squares fit to the standard Xxxxx energy level expression, yielding effective values for ωe and ωexe. The predicte...
