The effects of basis set selection on the vibrational transition frequencies obtained from SCF and correlated wave functions for an uncomplicated molecule, LiH

Abstract

The pattern of sensitivity to basis set selection in computed vibrational frequencies of LiH is examined through potential curve calculations at the SCF and at the well-correlated ACCD level. LiH is electronically uncomplicated in the sense that it has only four electrons, and so it affords the best opportunity to isolate basis set effects from lingering correlation features that might not be so completely accounted for in a more complicated molecule. With a very large basis of 64 functions, the transition frequencies are found to be within 2.7 cm−1 for ω0, within 3.3 cm−1 for ω12, and within 3.8 cm−1 for ω23 of measured values. Convergence with respect to basis set size is demonstrated by a decreasing sensitivity to basis enhancement as the largest set is approached. However, small sets can give excellent potential curves if the basis sets maintain a balance in the flexibility on the two atomic centers.