An ab initio investigation of the low-lying electronic states of BeH

Potential energy curves (PECs) for the ground state (X²Σ⁺) and the four excited electronic states (A²Π, B²Π, C²Σ⁺, ⁴Π) of BeH molecule are calculated using the multi-configuration reference single and double excited configuration interaction (MRCI) approach in combination with the aug-cc-pVTZ basis sets. The calculation covers the internuclear distance ranging from 0.07 nm to 0.70 nm, and the equilibrium bond length R_e and the vertical excited energy T_e are determined directly. It is evident that the X²Σ⁺, A²Π, B²Π, C²Σ⁺ states are bound and ⁴Π is a repulsive excited state. With the potentials, all of the vibrational levels and inertial rotation constants are predicted when the rotational quantum number J is set to be equal to zero (J = 0) by numerically solving the radial Schrödinger equation of nuclear motion. Then the spectroscopic data are obtained including the rotation coupling constant ω_e, the anharmonic constant ω_ex_e, the equilibrium rotation constant B_e and the vibration–rotation coupling constant α_e. These values are compared with theoretical and experimental results currently available, showing that they are in agreement with each other.