Full configuration-interaction study of the ionic–neutral curve crossing in LiF

Publisher Website
Google Scholar
Abstract
Full configuration‐interaction (FCI) calculations are used to assess the relative ability of methods for truncating the n‐particle expansion in describing the ionic–neutral curve crossing between the two lowest 1Σ+ states of LiF. While the FCI calculations yield a smooth dipole moment function, MRCI calculations based on CASSCF orbitals optimized for the lowest state at all r values yield a discontinuous dipole moment function. However, when the orbitals are optimized using a state‐averaged CASSCF procedure, with equal weights for the ionic and neutral solutions, both the CASSCF and MRCI dipole moment functions are smooth and in reasonable agreement with the FCI. No single‐reference‐based method is found to work satisfactorily. Potential curves for the lowest two 1Σ+ states are determined in both the adiabatic and diabatic representations using a large ANO Gaussian basis set and a state‐averaged CASSCF/MRCI treatment of electron correlation.