An optimal reduced space method for capturing the low-frequency motion in classical molecular dynamics calculations is presented. This technique provides a systematic means for carrying out reduced-dimensional calculations in an effective set of reduced coordinates. The method prescribes an optimal reduced subspace linear transformation for the low frequency motion. The method is illustrated with a dynamics calculation for a model potential, where the original six-dimensional space is reduced to two (three) dimensions, depending on the desired frequency cutoff value. (C) 2003 American Institute of Physics.