Subject: G98: How to extract force constants from output?

From: Peter Waaben Thulstrup <[email-masked]>
Date: Tue, 26 Feb 2002 15:58:15 +0100

Dear CCL

Can anybody give simple instructions on extracting stretching and
bending force constants for individual bonds from G98 output.

I hope to compare the computed values to force field parameters. Also,
if someone can recommend any key references or monographs on the subject
of adapting computed force constants for use in force fields I will be
most grateful for the help.

Thank you in advance,

Peter W. Thulstrup
Chemistry Department / Royal Veterinary and Agricultural University
Thorvaldsensvej 40 / DK-1871 Frederiksberg C / Denmark
Phone: +45 35282464 / Fax: +45 35282398 / E-mail: [email-masked]

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From: Christoph van Wüllen <[email-masked]>
Date: Tue, 26 Feb 2002 16:29:12 +0100

>bending force constants for individual bonds from G98 output.
... you can only "see" them in the checkpoint file, so use formchk to make
it human-readable, and then you see the force constant matrix in cartesian
coordinates.

To get stretch, bend etc. force coordinates, you have to define (in cartesian
coordinates) a displacement vector for this stretch, bend etc. Then the
force constant comes out of matrix-vector-algebra (using the cartesian
force const. matrix and the displacement vector).

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