Re: [AMBER-Developers] Origin of dihedral term

From: Jason Swails <>
Date: Tue, 20 Oct 2015 10:11:01 -0400

On Mon, Oct 19, 2015 at 9:22 PM, B. Lachele Foley <>

> "I'm sure she'd be happy to share her slides if she can find them and you
> wanted them. </throwing Lachele under the bus>"
> Not that I mean to avoid being tossed under a bus, and certainly neither
> do I wish to cause anyone else such discomfort, but... I just looked at my
> slides. Are you sure you meant me? From 2015?

​I am 100% sure that I meant you when I wrote that. :) But I think I'm
remembering differently since you clearly would know better than I whether
you gave that presentation -- I actually think it was Dave Cerutti that
gave that presentation now. But I *know* it was given (and given by
someone working on FFs, obviously), because I remember asking a question
about it. I also remember a slide where blindly fitting a Fourier series
without the n=0 term gave bad results and including the n=0 term gave a
better fit, precisely because there was an energy shift difference between
the MM and QM profiles. Another way of working around this, as Carlos
said, is to simply shift the QM profiles down to match where the MM profile
is scanning (basically, make the minimum energy of the QM scan 0, I
think). This seems easier to me than trying to center the QM scan around
the origin so you can fit it to cos(n*phi) instead of (1+cos(n*phi)).

Dave -- do you still monitor this list? Can you verify that I'm not losing
my mind? :)

But, without having time to think as deeply as I would like about it (and
> hoping Ross will... :-), your suggestion seems ok. I suppose it could add
> extra steps to the calculation, but perhaps a couple lines of code to
> reduce the terms could fix that, too.

​It shouldn't really add any steps to the calculation -- it's just adding
another term in the curve fit. But Adrian and Chad Hopkins worked out a
way to fit dihedrals using a linear least squares fit -- none of this
nonlinear stuff that can get stuck in a fake local minimum; something that
gives you the exact result incredibly quickly. I think they've published
this work as well. But if you just fit all terms from n=0 to n=6 (instead
of n=1 to n=6), your fit builds in an arbitrary potential shift.

I thought it was there to avoid having a negative correction in the torsion
> terms. This is what I told some students the other day... But, I'm not
> convinced that will necessarily be a problem. Re-tooling all the force
> fields, however, could require some work.

​All that removing the 1+ would do is introduce a constant shift in the
potential, which doesn't matter when it comes to computing forces and
thermodynamic quantities. But it *would* make Amber the black sheep of the
MD codes, which is a very good reason not to do it (of course it doesn't
matter how you *fit* the terms, so long as you're fitting a function with
the same gradients as the Amber potential everywhere).

All the best,

Jason M. Swails
Rutgers University
Postdoctoral Researcher
AMBER-Developers mailing list
Received on Tue Oct 20 2015 - 07:30:05 PDT
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