AMBER Archive (2006)

Subject: AMBER: A problem with Amber force field new parameter derivation

Date: Fri Feb 24 2006 - 00:16:04 CST

Dear Amber users,

        We are in the process of deriving Amber force field parameters
for a system of a transition metal atom coordinated to four other atoms
in square planar geometry:
        And we've taken a suitable xtal structure from the Cambridge
database containing this moieity, established what level of ab initio
theory and basis set reproduces the experimental geometry, calculated
the equilibrium geometry, and now want to compute the bond stretching
and angle bending force constants. The bond stretches (A-M, B-M, etc.)
were straightforward since each of the bonds can be changed independent
of the other, and we've computed the energy for a few values about the
equilibrium distance for each of the bonds and least squares fitted them
to k*x**2 to obtain the k's for each of the bonds separately.

        But, when it comes to the angle bends (A-M-B, B-M-C, etc.), we
can't change the value of one angle without affecting the other
angle(s), so the computed energy can no longer be fitted to a single
k*x**2 term. (i.e. Contributions come from more than one angle bend).
So, how does one go about determining each of the angle bend k's

        We'd appreciate any tips on how to tackle this problem.

Thanks very much.



Vidana C. Epa
Division of Molecular and Health Technologies,
343 Royal Parade,
Parkville, Victoria 3052,
tel.: (61) - 3 - 9662 - 7345
fax.: (61) - 3 - 9662 - 7347
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