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614. SIBFA: Sum of Interactions Between Fragments Ab Initio
Computed
by Nohad Gresh, Pierre Claverie and Alberte Pullman, Institut de Biologie Physico-Chimique, Paris, France with significant assistance from Jacqueline Langlet and Iannis Demetropoulos as well as many others on an on- going basis This rather large package is not a single program but a group of interacting programs which are intended to facilitate the calculation of interactions between fragments of molecules. One should be aware that the Ab Initio program from which starting information is obtained is not supplied with this package. The system which SIBFA currently operates with is IBMOL version H. It will be necessary to do a small amount of programming to interface it to another Ab Initio computational system such as GAMESS. SIBFA can quickly and reliably calculate both the intermolecular and intramolecular (conformational) energies which govern the binding specificities of large biomolecules.This procedure significantly differs from the standard molecular mechanics procedures in which the energies are computed by means of simplified Lennard-Jones type formulas and atomic point-charges to evaluate the electrostatic contribution. SIBFA relies on elaborate analytical formulas to compute the separate terms of the interaction energy. These formulas are derived by inferences from perturbation theory. Each molecular entity is built up from constitutive molecular fragments separated by single bonds for which Ab Initio calculations have been performed in advance. The variation of the intramolecular energy with conformation change is computed as the variable part of the sum of the interactions between the fragments. Some particularly distinctive features of this methodology are: * The electrostatic energy is computed by means of a multipolar expansion derived from the Ab Initio SCF molecular wave function. The multipoles (charges, dipoles and quadrupoles) are located on the atom centers and on the barycenters of the chemical bonds. * A polarization energy term is included in both inter- and intramolecular energy expressions. * The short-range repulsion is computed as a sum of bond-bond, bond-lone pair and lone pair-lone pair interactions. This was done in order to reproduce the variation of the Ab Initio short range repulsion as a function of the square of the overlap between orbitals located on the bond and lone pairs. This accounts for its radial and directionality dependencies. * A charge-transfer contribution is explicitly included in the intermolecular interactions. Several options are available which permit one to compute the different energy terms with varying degrees of accuracy, if needed, as is the case in the course of preliminary scannings of potential hypersurfaces. NOTE:This system is very large and will be distributed on an unlabeled, multifile magnetic tape with a block size of 4000 bytes. Each record will be 80 bytes. FORTRAN (IBM 3090) LInes of Code: 82,281 |