improper angle

From: Marco Sant (labicia_at_yahoo.it)
Date: Fri Nov 11 2011 - 02:34:42 CST

Dear all,
if in the parameters file I specify an improper angle
as formed by the following atoms:

A-B-C-D

does it mean that the improper angle is the one
between the planes A-B-C and B-C-D ??

I guess so looking at the meaning of dihedral angle,
A-B-C-D, as angle between planes A-B-C and B-C-D,
and seeing in the source code that dihedrals and impropers
have a similar approach.

Still, I have to answer to a referee for the publication
of an article and I need a RELIABLE answer.
I apologize if this is explained somewhere and it is
just me that cannot find it!

Secondary aspect. In an improper angle there
is always a central atom to which other three atoms
are connected.
In the lower schema the central atom is numbered as 1.

       3

Is there a restriction, when I specify the potential for the
improper A-B-C-D, to which position the central atom
should have?
For example, in my calculations I specify the improper
of the above schema as: 2-3-1-4.
In other words: should the central atom 1,
appear in position B and C, or in position A and D,
when specifying A-B-C-D improper?

Thanks a lot,
Marco and Bin

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