A new approach is proposed to extract the fundamental absorption frequency from classical molecular dynamics simulation in which the oscillator amplitude is specified by setting the total energy equal to the harmonic zero-point energy. This approach is applied to D-2 molecule. The fundamental absorption frequency and the anharmonic constant of D-2, obtained from the quasi-classical direct ab initio molecular dynamics simulation method with the full Configuration Interaction level of theory with the aug-cc-pVTZ basis set, compare well with the experimentally observed values. (C) 2004 Elsevier B.V. All rights reserved.