The approximation of the full nuclear potential of a molecular system in its equilibrium geometry
and in the vicinity of the respective minimum on the potential energy surface by
are mass-weighted cartesian displacements of nuclei relative to their equilibrium
positions. The approximation allows one to describe vibrational motion in terms of
independent vibrational modes (normal modes) each of which is governed by a simple
one-dimensional harmonic potential.
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook