Ab initio molecular force fields fitted in Cartesian coordinates to experimental frequencies of isotopic species using symmetry constraints: application to indole and pyrrole molecules

Abstract

We present the new computational procedure for the fitting ab initio molecular force fields represented in Cartesian coordinates to the experimental frequencies with inclusion data for isotopic species and symmetry constraints. This procedure is based on the original numerical algorithms developed within theory of regularization of ill-posed problems and included in the software package SPECTRUM. New procedure allows organize the synthesis of molecular force field of the target molecule from the matrices in Cartesian coordinates of corresponding patterns and to avoid difficulties with introducing internal coordinates for a case of complicated molecules, so appears beneficial for fitting vibrational spectra of the large molecular systems, when only moderately accurate quantum chemistry methods may be applied. In this work, an earlier suggested algorithm for scaling force matrices in Cartesian coordinates has been extended to allow direct reduction by symmetry and simultaneous use of the experimental data on several isotopic species of a molecule. The molecules of pyrrole and indole (typical fragments of biological molecules) were chosen to demonstrate the efficiency of scaling procedure in Cartesian coordinates with symmetry constraints.