Accurate Atomic and Molecular Calculations without Gradient Corrections: Scaled SVWNV Density Functional.

The local spin density approximation (LSDA) approximation was one of the first density functional theory (DFT) methods employed to calculate atomic and molecular properties. As newer, more sophisticated methods, such as BLYP and B3LYP, were developed, the LSDA approximation has grown less popular for molecular systems. In this paper we revisit the LSDA method and investigate a simple way to improve the results that can be obtained using this approximation. By scaling the contribution of the local correlation to the SVWNV functional, improved results can be obtained for heats of formation, ionization potentials, electron affinities, bond angles, bond distances, vibrational frequencies, conformational energies, interaction energies, and barrier heights. The results of our studies show that scaling the SVWNV functional yields heats of formations with average unsigned errors up to about nine times smaller than those of the standard SVWNV functional. The decreases in the errors of other properties studied in this work were not as dramatic as those of the heat of formation but were, in most cases, significant. There is a notable time saving in this density only functional. For a 9-alanine system SVWNV is 55% faster than B3LYP and 40% faster than BLYP at a 3-21G* basis set. Based on our observations we propose an improved SVWNV density functional that is suitable for the study of molecular systems at a fraction of the cost of more sophisticated DFT methods, which also produces reasonable accuracy at small basis sets. One type of application for which the improved SVWNV functional would be extremely well suited is QM/QM methods where a fairly inexpensive method is needed for the larger part of a system that is treated at a lower level of theory.