Obviously this all goes exactly the same as for bond lengths... EH looked in the CSD for
bond angles in amino acids and amino acid-like molecules. They determined for each type
of bond the ideal bond angle and the standard deviation, like 109.4 +/- 3.3 or 120.1 +/- 1.3 etc.
Their force field now simply is a list of all groups of three atoms that are A-B-C connected
with the associated angle and SD. This FF gac be used to 1) optimize a structure by moving atoms
to optimize all angles (using the SD as some force constant) and 2) validate structures by
enumerating how many SDs (remeber: the number of SDs that a value deviates from the ideal
value is called the Z-score)each angle deviates from that ideal value.
A little known simple fact about normal distributions is that the RMS Z-score (i.e. the
RMS of all Z-scores is 1 if the errors in the molecule have a normal distribution with the
'correct' average and SD.