Interaction matrix

After skipping a line in rbdata, include the interaction matrix using syntax

matrix
[class i] [class j] [potential name] [parameters]
[class i] [class j] [potential name] [parameters]

where the potential names and parameters are specified below. The interaction matrix is assumed to be symmetric, so if $(i,j)$ is specified, $(j,i)$ should not be specified.

SANDBOX will compute certain values that pertain to individual sites only if those sites interact with certain potentials. For example, if a site interacts with some other site according to a dipole interaction, then $\hat{\mu}$ must be computed for both sites. If the row (or column) corresponding to a site's interaction contains only empty or isotropic interactions, however, $\hat{\mu}$ is not computed.