Scalar J-Coupling Restraints

X-PLOR has an energy term for fitting scalar couplings to the Karplus relation.

$$J = A \cos^2 (\theta+P) + B \cos (\theta+P) + C.$$ (22.1)

where $\theta$ is the dihedral angle for the three-bond coupling, $A$, $B$, and $C$ are so-called Karplus coefficients, and $P$ is a phase.

The refinement energy term is $E_{COUP} = K_{COUP}(J-J_{obs})^2.$ The reference is Garrett et al. (1994).