AMBER Archive (2008)

Subject: Re: AMBER: question about the improper torsion

From: Bill Ross (
Date: Tue Jul 15 2008 - 11:48:24 CDT

> there are improper torsions as X-X-C-D
> how do we know it should be A-B-C-D or B-A-C-D, for the two have
> different improper dihedrals respectively?

I think alphabetical ordering is used, probably by atom type
but possibly by atom name. Speaking as the person who coded it
in leap originally.

Only planar impropers should be defined. This rules out using
impropers to set non-planar geometry in united atom force fields.

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