HARMONIC AND QUASIHARMONIC DESCRIPTIONS OF CRAMBIN

We report a normal-mode analysis of crambin using various versions of the CHARMM, AMBER, DISCOVER, and AMBER/OPLS force fields, comparing static and dynamic features derived from these commonly used potential energy functions. We have also carried out a quasiharmonic analysis of crambin using a 100-ps dynamics simulation with the CHARMM potential energy function. Although the detailed character of the lowest frequency modes is sensitive to the force field used (and is different for the quasiharmonic and harmonic results), many general features of the frequency distributions and computed fluctuations are remarkably similar in the various calculations. The density of states in the 0-200-cm-1 region agrees well in the quasiharmonic and harmonic calculations, but only for sampling times greater than about 50 ps; for shorter times, the density of certain low-energy vibrations is significantly underestimated. Trial calculations with increased dielectric constants or in the presence of a spherical shell of water atoms show fluctations markedly different from any of the "standard" vacuum calculations. Overall, these results provide new evidence about the insensitivity of computed dynamical properties to the assumed potential function and about the convergence of quasiharmonic molecular dynamics simulations. © 1990 American Chemical Society.