Each of the dihedral interaction classes defines a potential that depends upon the dihedral angle involving two bonds that connect four atoms. Each such group of atoms is assigned an dihedral type index, and different values of the parameters in the dihedral potential function are used for different dihedral types.

All of the dihedral interaction classes implement a common interface, which is described here.

Geometry

Let the three atoms in an dihedral group be labelled 0, 1, 2, 3, with bonds connecting 0-1 and 1-2. Let ${\bf r}_{i}$ be the position vector for atom i, with $0 \leq i < 3$ . We define three bond vectors

${\bf b}_{1} \equiv {\bf r}_{1} - {\bf r}_{0} \quad\quad\quad {\bf b}_{2} \equiv {\bf r}_{2} - {\bf r}_{1} \quad\quad\quad {\bf b}_{3} \equiv {\bf r}_{3} - {\bf r}_{2}$

The dihedral potentials are all function of the dihedral angle $\phi$ . This angle $\phi$ is the angle between the plane spanned by ${\bf b}_{1}$ and ${\bf b}_{2}$ the plane spanned by ${\bf b}_{2}$ and ${\bf b}_{3}$ . It is defined so that a planar cis configuration corresponds to $\phi = 0$ , and a trans zigzag configuration corresponds to $\phi = \pi$ . A more explicit expression for $\cos(\phi)$ is given in Dihedral Interactions.

Dihedral Interaction Classes

CosineDihedral - proportional to cosine of dihedral angle
MultiHarmonicDihedral - Fourier expansion in dihedral angle