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.