Accurate vibrational frequencies using the self-consistent-charge density-functional tight-binding method

An optimization technique for enhancing the quality of repulsive two-body potentials of the self-consistent-charge density-functional tight-binding (SCC-DFTB) method is presented and tested. The new, optimized potentials allow for significant improvement of calculated harmonic vibrational frequencies. Mean absolute deviation from experiment computed for a group of 14 hydrocarbons is reduced from 59.0 to 33.2 cm−1 and maximal absolute deviation, from 436.2 to 140.4 cm−1. A drawback of the new family of potentials is a lower quality of reproduced geometrical and energetic parameters.