Re: amber-developers: more on linear angles.

From: David A. Case <>
Date: Mon, 14 Jun 2004 18:08:36 -0700

On Mon, Jun 14, 2004, James W. Caldwell wrote:

> I setup a simple test: F-O-F, linear, with a linear force constant.
> the angle energy should be zero, right? well it's 0.2 kcal/mole
> in both sander7 and sander8. I've attached a tar file with an input,
> prmtop, prmcrd and the sander8 output.

See the code around line 416 of ene.f, and consider changing "pt999"
to something closer to unity. Note that acos(-0.999) is 177.4 degrees,
is significantly from 180. There is a nasty problem involved in getting
good forces with linear bond angles. Someone should plunge in and fix the
code (similar problems arise with linear angles in ephi.f). I have a
vague recollection that moil does this "correctly", so that might be worth


David A. Case                     |  e-mail:
Dept. of Molecular Biology, TPC15 |  fax:          +1-858-784-8896
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Received on Wed Apr 05 2006 - 23:50:01 PDT
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