It is computed by comparing the LLG values from the translation
search with LLG values for a set of random translations. 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