Localized Normal Coordinates in Accurate Vibrational Structure Calculations: Benchmarks for Small Molecules.

The impact of localized CH-stretching normal coordinates in comparison to canonical normal coordinates on the performance of accurate vibrational structure calculations has been studied for simple molecules of up to eight atoms. Two aspects have been considered in detail, (a) the (pre)screening of coupling terms within an n-mode expansion of the multidimensional potential energy surface and (b) the demands in vibrational configuration interaction calculations (VCI). All calculations have been performed in a realistic setup, and the effect of any approximation has been measured in deviations of the final VCI frequencies, which allows for a direct comparison with experimental data.