The inverse mass record contains inverse masses [amu^-1] for each of the atoms in the system. The format of the record is:

ivms	Natom
	inversemass1
	...

Following the keyword is the number of atoms N_atom. Although the number of atoms is already declared in the header of the descriptor file, you can actually specify a different number here as long as it is as large as the number of atoms. Following this are N_atom inverse masses inversemass₁, inversemass₂, and so on. Note that a value of 0.0 indicates an infinitely heavy atom. If you have such atoms, all simulations will fail from numerical overflow. Example:

ivms 5

0.08333
1.0
1.0
1.0
1.0

An example containing five inverse masses. The first inverse mass of 0.08333 corresponds to a mass of 12 amu (carbon perhaps?) and the remaining four numbers correspond to a mass of 1 amu (hydrogen?)