High Quality Content by WIKIPEDIA articles! X-PLMolecular mechanics uses Newtonian mechanics to model molecular systems. THe potential energy of all systems in molecular mechanics is calculated using force fields. MOlecular mechanics can be used to study small molecules as well as large biological systems or material assemblies with many thousands to millions of atoms. OR Or XPLOR-NIH is a software package for computational structural biology originally developed by Axel Brunger at Yale University. IT was first published in 1987 as an off-shoot of CHARMM - a similar program that ran on Cray supercomputers. IT is used in the fields of X-ray crystallography and NMR analysis. VAriations on this theme are possible; for example, many simulations have historically used a "united-atom" representation in which methyl and methylene groups were represented as a single particle, and large protein systems are commonly simulated using a "bead" model that assigns two to four particles per amino acid.