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Re: [ESPResSo-users] Triple Cos term formula for Dihedral potential

From: Axel Arnold
Subject: Re: [ESPResSo-users] Triple Cos term formula for Dihedral potential
Date: Thu, 26 May 2011 21:25:36 +0200
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On Thursday 26 May 2011 15:53:02 Muhammad Anwar wrote:
> Hi,
> Dear Dr Arnold,
> I am modifying the dihedral potential code, I am not understanding how
> and why you divide the equation for force by sin(phi)
> sinmphi_sinphi = sin(iaparams->p.dihedral.mult*phi -
> iaparams->p.dihedral.phase)/sin(phi);

I also didn't write the code, but from a first glimpse, it looks like the 
vectors are not normalized, and the 1/sin(phi) is doing that.
> Secondly, when sin(phi) is small how you get this equation
> sinmphi_sinphi = iaparams->p.dihedral.mult*
>       cos(iaparams->p.dihedral.mult*phi -
> iaparams->p.dihedral.phase)/cosphi;

See the comment in the code, it is simply an McLaurin-expansion for small phi.

Otherwise, I think it is important that you understand the math of the 
dihedral potential. By this I mean also how to get the dihedral angles and 
calculate the forces. That is especially important, since the definitions of 
the angles are not unique, and you need to match them to what you need.
Many regards,

Axel Arnold
Martha-Schmidtmann-Str. 7       Phone:  +49-173-8706659
D-70374 Stuttgart               E-Mail: address@hidden

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