Perform hydrogen mass repartitioning.

hmassrepartition [parm <name> | crdset <set> | parmindex <#> | <#>] [<mask>] [hmass <hydrogen new mass>] [dowater]

parm <name> Modify topology selected by name.
crdset<set> Modify topology of COORDS set.
parmindex <#> | <#> Modify topology selected by index <#> (starting from 0).
<mask> Atoms to modify (all solute atoms by default).
hmass <hydrogen new mass> Mass to change hydrogens to (3.024 u by default).
dowater If specified, modify water hydrogen mass as well.

Perform hydrogen mass repartitioning on the specied topology. Hydrogen mass repartitioning means that for a given heavy atom, the mass of all bonded hydrogens are increased (to 3.024 u by default) and the mass of that heavy atom is decreased so as to maintain the same overall mass. The main use case is to allow longer time steps for molecular dynamics integration due to reduced frequency of vibration of bonds to hydrogen atoms.