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. (c) 2005 Elsevier B.V. All rights reserved.