Simulation of B state

GROMACS version: 2020.3
GROMACS modification: No

Hello GROMACS team,

I prepared a hybrid topology with pmx and am wondering how to simulate the B state only. Does the following combination of mdp options achieve this?

free_energy = no
init_lambda = 1

I already checked whether this produces a difference in total energy in the first place (which it does). Alternatively, is there a way to avoid the performance loss if one turns on the free_energy flag?

Thanks in advance,