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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
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