Dihedral styles and sign convention

Dear everybody,

I checked how the dihedral angle is calculated compared
to the sign convention in the documentation. The variable
phi in Opls uses the opposite sign convention. The same
code is also used in Helix and quadratic dihedral styles.

For the Opls, only cos(n*phi) enters the Hamiltonian, so it
doesn't matter, but for the two other styles the Hamiltonian
does depend on the choice of convention.