MacroModel Reference Manual - ISP

Chapter 4: Operation Codes>0 Specify fixed or frozen status for the atom specified; this can override a previousspecification for the same atom.arg4Half width of a flat bottom restraintSpecified in tenths of an Angstrom (an integer).arg5 Force constant (kJ mol -1 Å -2 )The units of the force constant are configurable by means of the FIXmultiplier in the force-field file in use. Internally, the program useskJ mol –1 as its energy unit, so that, after multiplication by the multiplier,FXAT force constants have these units. Most force-field files we supplyspecify a FIX multiplier of 4.184. Thus, the units given in the .com file,and in the Maestro interface, correspond to kcal/mol for these forcefields. In other words, the default versions of the force field files forAMBER*, MM2*, MM3*, OPLS, and MMFF indicate that fixed-atomconstraints have the units of kcal mol -1 Å -2 , but the units of fixed-atomconstraints for the OPLS_2001 force field are kJ mol -1 Å -2 .0 500.0 (default).>0 Use this value instead of default.