Intermolecular vibrational relaxation, picosecond

Molecular dynamics is a true first principles dynamic molecular model. It simply solves the equations of motion. Given an intermolecular potential, MD provides the exact spatial and temporal evolution of the system. The stiffness caused by fast vibrations compared with slow molecular relaxations demands relatively small time steps and challenges current simulations. As an example, the time scale associated with vibrations is a fraction of a picosecond, whereas those associated with diffusion or reaction may easily be in the seconds to hours range depending on the activation energy. Consequently, MD on a single processor is usually limited to short time and length scales (e.g., pico- to nanoseconds and 1-2 nm). 

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