A systematic study of the convergence and additivity of correlation and basis set effects on the force constants of small molecules: HF, HCN, and NH3

Abstract

The quadratic and cubic force field, as well as the dominant quartic force constants of HF, HCN, and NH3 have been evaluated ab initio, using SCF, CI (singles and doubles) and cluster-corrected CI wave functions. Five different basis sets have been used, ranging in quality from small double-zeta to triple-zeta with two sets of polarization functions. Our main conclusions are as follows: (1) Transferability of the correlation contribution between different basis sets is quite poor; in particular, basis sets without polarization functions yield exaggerated correlation contributions. (2) Basis set and correlation effects on the force constants can be separated into an intrinsic and a geometry contribution, with the latter dominating for stretchings. Using an accurate reference geometry (e.g., from a high-quality calculation) in conjunction with a less accurate wave function for the evaluation of the force fields yields improved results. (3) Correlation contributions to cubic and quartic force constants, particularly to stretchings, are small. These quantities can, therefore, be evaluated at the less costly SCF level. (4) Double-zeta-plus polarization basis sets may have quite significant residual basis set errors at the SCF level; these errors were generally attributed to correlation effects in the past.