From adiabatic to diabatic representation: an ab initio investigation of the LiS molecule

In this study, we present a high-level theoretical investigation of the electronic structure and nonadiabatic dynamics of the LiS molecule. High-precision adiabatic potential energy curves (PECs) for the lowest electronic states were computed using multireference configuration interaction with Davidson correction (MRCI+Q). In addition, we determine the permanent (PDMs) and transition (TDMs) dipole moments, as well as the radiative lifetimes of vibrational levels to characterize the molecule's spectroscopic and dynamical properties. The nonadiabatic radial coupling ab initio matrix elements were obtained from the ab initio adiabatic PECs using the finite difference method between the (X²Π-2²Π) and (2²Π-3²Π) adiabatic states. The first-order adiabatic correction was also evaluated, and its effects on the spectroscopic constants and vibrational energy levels were analyzed for both the X²Π and 2²Π states. This work advanced the theoretical framework for modeling diatomic systems in which nonadiabatic effects may be significant, with implications for precision spectroscopy and molecular dynamics.