Our least-squares fitting procedure, which jointly uses the energy and gradient data obtained from DFT calculations, is applied to construct the anharmonic force field of acetonitrile. The quartic force field performed at B3LY-P/cc-pVTZ level of theory from 101 simplex-sum energy and gradient data show very good agreement in comparison with the classical 710 energy calculations. Vibrational energy transitions obtained from the two force fields in the range of 300-3000 cm(-1) are consistent with each other. (C) 2004 Elsevier B.V. All rights reserved.