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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 |

**User-agent**: |
KMail/1.12.2 (Linux/2.6.31-23-generic; KDE/4.3.2; i686; ; ) |

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