next up previous contents index
Next: Coulomb restraint Up: Restraints and their derivatives Previous: Upper bound   Contents   Index

Cosine restraint

This is usually used for dihedral angles $f$:


\begin{displaymath}
c = \vert b \vert - b \cos(n f + a)
\end{displaymath} (7.56)

where $b$ is CHARMM force constant, $a$ is phase shift (tested for 0 and 180${}^{o}$), and $n$ is periodicity (tested for 1, 2, 3, 4, 5, and 6). The CHARMM phase value from the CHARMM parameter library corresponds to $a-180$${}^{o}$. The force constant $b$ can be negative, in effect offsetting the phase $a$ for 180${}^{o}$ compared to the same but positive force constant.


\begin{displaymath}
\frac{ \; {d}c}{ \; {d}f} = b n \sin(n f + a)
\end{displaymath} (7.57)



Bozidar BJ Jerkovic 2001-12-21