Scaling of ab initio force fields by MOLVIB

Abstract

Nowadays, it is possible to perform ab initio calculations on relatively large molecules. The vibrational frequencies obtained in this way differ from the observed frequencies, partly because of anharmonicity, and partly because of approximations in the quantum mechanical methods used (neglect of electron correlation and basis set truncation). Because of this, ab initio force fields generally have to be scaled using spectroscopic data. A new version of MOLVIB, a program for vibrational force field calculations, has recently been prepared. With this program, it is now possible to perform scale factor calculations according to several methods, which are described. Results of application to some small molecules are also presented.