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514. BIGSTRN-3: General-Purpose Empirical Force-Field
Program
by Robert B. Nachbar, Jr., Merck Sharp & Dohme Research
Laboratories, Rahway, New Jersey 07065 and Kurt Mislow,
Chemistry Department, Princeton University, Princeton,
New Jersey 08544
BIGSTRN-3 is a general-purpose empirical-force-field
(EFF) program. Its primary goal is to optimize a given
molecular geometry in a given force field, possibly
under a user-defined set of constraints, to the nearest
stationary structure. Ancillary operations include
heat of formation calculation, calculation of
thermodynamic functions, normal mode analysis, and
tracing of conformational reaction pathways.
Four empirical force fields (Andose-Mislow (AM),
Engler-Andose-Schleyer (EAS), Allinger (MM2), and
Ermer-Lifson (CFF)) are provided with the program in
the form of individual data files. Because most of the
force field is defined in the data file, BIGSTRN-3 has
maximum independence from any particular force field.
Additional parameter data files can be searched at run
time to resolve undefined interactions.
There are a number of geometry optimization schemes
available to the user, in which either the energy or
the gradient norm of the energy (the forces) can be
minimized. These optimizers use the steepest-descent,
conjugate-gradient, variable-metric or Newton-Raphson
methods. Analytical first and/or second derivatives of
the energy or the gradient norm of the energy with
respect to Cartesian coordinates are used throughout
the program. The calculation of transition states
(single partial maxima) and higher order partial maxima
is easily achieved with BIGSTRN-3; these stationary
structures can be calculated directly, and one does not
need to resort to complex "driving" techniques. Upon
convergence, the matrix of analytical second
derivatives of the energy is evaluated and
diagonalized. From the number of negative eigenvalues,
one can determine the nature of the stationary
structure obtained: zero, a minimum (M(0)); one, a
single-partial maximum (M(1)); two, a double-partial
maximum (M(2)); etc.
The imposition of geometric constraints has been
enhanced to include distances, bond angles, torsion
angles, and out-of-plane angles.
Comprehensive diagnostic messages are provided for user
input errors. The format of the printed output has
been modified to provide the user the maximum amount of
information about the force field used, the given
molecule, and the geometry optimization in a form as
unambiguous and clear as possible.
FORTRAN 77 (VAX)
Lines of Code: 25,000
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