HARMONIC-ANALYSIS OF LARGE SYSTEMS .1. METHODOLOGY

Methods have been developed for the determination of vibrational frequ encies and normal modes of large systems in the full conformational sp ace (including all degrees of freedom) and in a reduced conformational space (reducing the number of degrees of freedom). The computational method, which includes Hessian generation and storage, full and iterat ive diagonalization techniques, and the refinement of the results, is presented. A method is given for the quasiharmonic analysis and the re duced basis quasiharmonic analysis. The underlying principle is that f rom the atomic fluctuations, an effective harmonic force field can be determined relative to the dynamic average structure. Normal mode anal ysis tools can be used to characterize quasiharmonic modes of vibratio n. These correspond to conventional normal modes except that anharmoni c effects are included. Numerous techniques for the analyses of vibrat ional frequencies and normal modes are described. Criteria for the ana lysis of the similarity of low-frequency normal modes is presented. Th e approach to determining the natural frequencies and normal modes of vibration described here is general and applicable to any large system . (C) 1995 by John Wiley & Sons, Inc.double dagger