The POTEntial statement allows one to select specific
restraining functions for particular classes of distance
restraints. By partitioning
the distance restraints into particular classes, several
restraining functions can be mixed. The total distance
restraint energy 
is a sum over all classes of
distance restraints:
where the second sum is carried out over all distance restraints
of the particular class. Each restraint corresponds to exactly one
ASSIgn statement except for the SYMM, 3DPO, and HIGH restraining
functions, where each restraint can comprise groups of ASSIgn statements.