It is computed by comparing the LLG values from the rotation
search with LLG values for a set of random rotations. The mean and
the RMS deviation from the mean are computed from the random set,
then the Z-score for a search peak is defined as its LLG minus the
mean, all divided by the RMS deviation. Read, R.J. (2001). "Pushing
the boundaries of molecular replacement with maximum likelihood."
Acta Cryst. D57, 1373-1382