Accurate Atomic and Molecular Calculations without Gradient Corrections: Scaled SVWNV Density Functional Kevin E. Riley, Edward N. Brothers, Kenneth B. Ayers, and Kenneth M. Merz* Department of Chemistry, The PennsylVania State UniVersity, 104 Chemistry Building, UniVersity Park, PennsylVania 16802 Received January 12, 2005 Abstract: 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. Introduction As the speed of modern computers increases while their cost decreases it becomes possible to consider the use of ab initio methods to calculate properties of very large molecules, such as biomolecules. These types of calculations promise to greatly increase our understanding of the structure and function of large molecular systems. 1 Although a few efforts have already been made to make such large scale calcula- tions, 2-6 it is not yet a common practice due to the great computational expense. 1 The first step one would take when making ab initio calculation on very large systems would be to use a relatively inexpensive density functional theory (DFT) method, 7,8 such as LSDA, along with a small split valence basis set with a low degree of contraction, such as 3-21G*. 9-11 The problem with such an approach is that the LSDA method is known to give inaccurate results when used to calculate such properties as the heat of formation. 12,13 In this study we use an empirical technique to scale the correlation (VWNV) 14 part of the SVWNV functional in order to improve the overall accuracy of several atomic and molecular properties as calculated using the LSDA method. This approach was initially described by Brothers and Merz, and we have expanded on the initial observation herein. 13 * Corresponding author phone: (814)865-3623; e-mail: merz@psu.edu. 546 J. Chem. Theory Comput. 2005, 1, 546-553 10.1021/ct050007c CCC: $30.25 © 2005 American Chemical Society Published on Web 06/02/2005