# VisitModelInnerPatch¶

class VisitModelInnerPatch : public feasst::VisitModelInner

Patch interactions are defined by two sites. These are called centers or directors.

The center site is used in the computation of the separation vector. Thus, for a Kern-Frenkel model, the centers would be the hard spheres.

The director site is used to determine the orientation of the patch.

The visitor should loop over a group_index which contains center sites only. Each pair of center sites is then subject to a cutoff which should be the maximum cutoff for all possible pairs of director types between the two centers. Once two centers are said to be within the cutoff, then all pair director interactions are computed. Directors are identified as those sites which are bonded to the center and whose bond is identified as a director bond.

The orientational component of the patch is determined by this visitor class. But the distance-based component is determined by the model. The types for the model computation are determined by the directors. E.g., the sigmas, epsilons, cutoffs etc are set by the directors.