The low-lying electronic states of BeP: a reliable and accurate quantum mechanical prediction

A very high level of theoretical treatment (complete active space self-consistent field CASSCF/MRCI/aug-cc-pV5Z) was used to characterize the spectroscopic properties of a manifold of quartet and doublet states of the species BeP, as yet experimentally unknown. Potential energy curves for 11 electronic states were obtained, as well as the associated vibrational energy levels, and a whole set of spectroscopic constants. Dipole moment functions and vibrationally averaged dipole moments were also evaluated. Similarities and differences between BeN and BeP were analysed along with the isovalent SiB species. The molecule BeP has a X 4Σ- ground state, with an equilibrium bond distance of 2.073 A, and a harmonic frequency of 516.2 cm-1; it is followed closely by the states 2Π (Re = 2.081 A, ωe = 639.6 cm-1) and 2Σ- (Re = 2.074 A, ωe = 536.5 cm-1), at 502 and 1976 cm-1, respectively. The other quartets investigated, A 4Π (Re = 1.991 A, ωe = 555.3 cm-1) and B 4Σ- (Re = 2.758 A, ωe = 292.2 cm-1) lie at 13 291 and 24 394 cm-1, respectively. The remaining doublets (2Δ, 2Σ+(2) and 2Π(3)) all fall below 28 000 cm-1. Avoided crossings between the 2Σ+ states and between the 2Π states add an extra complexity to this manifold of states.