Calculation of molecular configuration integrals

A method is presented for calculating the conformational free energy of a molecule in all degrees of freedom. The method uses the harmonic approximation with finite integration ranges, along with Mode Scanning, a fast correction for anharmonicity based upon internal bond-angle-torsion coordinates. Mode Scanning accounts for local anharmonicity without the need for expensive Monte Carlo integration. The method is efficient, and comparisons with analytic or highly detailed numerical calculations show excellent accuracy. Similar comparisons for the previously described Mode Integration method show that, although it is computationally demanding, it can be less accurate than the pure harmonic approximation. The inaccuracy of Mode Integration is traceable to its use of a Cartesian coordinate basis set; much more accurate results are obtained when the basis set consists of bond-angle-torsion coordinates.