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Re: [lammps-users] Angle Harmonic Gradients question
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Re: [lammps-users] Angle Harmonic Gradients question


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Wed, 18 Oct 2017 10:27:24 -0400

On Tue, Oct 17, 2017 at 9:32 AM, Matthew Wander <mcfwander@...24...> wrote:
> I have a question about lines 115-120 of angle_harmonic.cpp. I would have
> thought the gradients would have been a series of cross products of the bond
> vectors: r12 x (r32 x r12) appropriately normalized multiplied by the force
> constants etc. . However this is something very different. I tried checking
> the manual for a reference but there was none. What is going on here?

i didn't have the time to work though the code in detail, so here is
just an educated guess:

it looks to me as if the angle force is decomposed into pairwise force
components in the molecule centered frame with the center of mass
being the origin:
one force component that moves the two outer atoms as if there was a
bond between them, and a second component that moves them up or down
with the central atom moving in the opposite direction of the outer
atoms.
the rest looks like projecting these force components on the real
space distance vectors in a similar fashion as it is done with
pairwise forces.

axel.

>
> thanks
> matthew
>
>
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-- 
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.