The fundamental frequencies and the mean values of the internal coordinates are extracted from the trajectories of classical dynamics based on the relation between the classical and the quantum mechanical frequencies using quasi-classical direct ab initio molecular dynamics, where the oscillator amplitude is specified by setting the total energy equal to the harmonic vibrational energy. This method is applied to a triatomic molecule H2O. The harmonic frequencies, the fundamental absorption frequencies, and the mean structures are obtained in good agreement with experimentally observed values with the theoretical level of MP2 using the aug-cc-pVTZ basis set. (c) 2007 Elsevier B.V. All rights reserved.