The molecular structure must be defined, the parameters read in, and the distance and dihedral restraints defined by the desired NOE (Chapter 18) and restraints dihedral (Section 7.2) statements before the MMDG statement can be invoked. If rigid groups are to be defined in the MMDG initialize statement, the coordinates after which the groups are modeled must be defined in the main coordinate set. The reference coordinate set has to be defined for the pseudoatom correction and generation of conformational distance bounds if REFErence=COORdinates is specified. The atom selection for the DG energy term is fragile (Section 2.15).