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     MASS int word real [ACCEptor]
                        [DONOr   ]

The MASS command specifies the chemical types of atoms, their names, their masses, and optionally whether they are hydrogen bond donors or acceptors. This command is one of the most important in the topology file because is specifies all the permissible atoms in any system.

The int is the numerical chemical type code as used in the parameter file, see Param Files. Its value may not exceed the parameter MAXATC, which is currently 100. The word is the chemical type name, and this symbol is used in the parameter file. It can also be referenced when analysis tables are built, see Build Syntax. The real number specifies the mass of each atom type in Atomic Mass Units. Finally, the optional keywords, ACCEPTOR and DONOR, indicate when the atom can participate in a hydrogen bond as an acceptor or donor, respectively. These finally keywords are used only by the RTF GENERATE command, see RTF Generate Command.