New users always seem to assume a zero-step MD is a single-point energy, but this is only true with unconstrained-start having the right value.
grompp should warn when the erroneous combination is used.
grompp should note in either case that mdrun -rerun should be used.
IIRC zero-step EM already does some similar output. Not sure whether that is in mdrun or not - if so, move to grompp.
Warn if using nsteps=0 without continuation=yes
To achieve true single-point energy evaluations for a structure
we should not constrain the initial conformation. This is
typically done by settings nsteps=0, so grompp now warns if this
is used while keeping continuation = no (the default).