gmx convert-tpr [-s [<.tpr/.gro/...>]] [-n [<.ndx>]] [-o [<.tpr/.gro/...>]] [-extend <time>] [-until <time>] [-nsteps <int>] [-[no]zeroq]
1. by modifying the number of steps in a run input file with options -extend, -until or -nsteps (nsteps=-1 means unlimited number of steps)
2. by creating a .tpx file for a subset of your original tpx file, which is useful when you want to remove the solvent from your .tpx file, or when you want to make e.g. a pure Calpha .tpx file. Note that you may need to use -nsteps -1 (or similar) to get this to work. WARNING: this .tpx file is not fully functional.
3. by setting the charges of a specified group to zero. This is useful when doing free energy estimates using the LIE (Linear Interaction Energy) method.
Options to specify output files:
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