Hamiltonian REX: different exchange probability with -nb gpu

GROMACS version: 2019.4
GROMACS modification: Yes (PLUMED 2.5.7 patched)

Dear all,
I am testing the hamiltonian replica exchange for a small RNA tetraloop system (NVT condition, hydrogen mass repartition 4 fs time step). As a part of testing, I am using the same scaled topology for four replicas to check exchange probability. The test run was performed for 400 ps. However, when I use the “-nb gpu” option with mdrun, I get a slightly low exchange rate. The simulations are repeated and the results are consistent.
Also, replex (250) is set as multiple of nstlist (25) and dynamic load balancing is also off (-dlb no).

Please, can you suggest whether using “-nb gpu” could affect energies and thus exchange probability in general?

mdrun command line:
mpirun -np 4 gmx_mpi mdrun -v -deffnm md -plumed plumed.dat -replex 250 -hrex -multidir dir_0 dir_1 dir_2 dir_3 -dlb no -nb gpu -nsteps 100000 >& log

With “-nb gpu”:
dir_0/md.log:Repl average probabilities:
dir_0/md.log-Repl 0 1 2 3
dir_0/md.log-Repl .96 .84 .97
dir_0/md.log-Repl number of exchanges:
dir_0/md.log-Repl 0 1 2 3
dir_0/md.log-Repl 190 172 192
dir_0/md.log-Repl average number of exchanges:
dir_0/md.log-Repl 0 1 2 3
dir_0/md.log-Repl .95 .86 .96

without “-nb gpu”:

…/No_nb_gpu/dir_0/md.log:Repl average probabilities:
…/No_nb_gpu/dir_0/md.log-Repl 0 1 2 3
…/No_nb_gpu/dir_0/md.log-Repl .96 .97 .97
…/No_nb_gpu/dir_0/md.log-Repl number of exchanges:
…/No_nb_gpu/dir_0/md.log-Repl 0 1 2 3
…/No_nb_gpu/dir_0/md.log-Repl 198 192 197
…/No_nb_gpu/dir_0/md.log-Repl average number of exchanges:
…/No_nb_gpu/dir_0/md.log-Repl 0 1 2 3
…/No_nb_gpu/dir_0/md.log-Repl .99 .96 .99

Calculating the nonbonded interaction on gpu or not should not affect the results.
If you are running on the same machine and GROMACS is compiled accounting for gpu the non-bonded interactions are calculated on GPU with and without explicitly add the option -nb gpu).
My guess is that the difference you observed are due too not enough sampling.
Best regards

Hi Alessandra,
Thanks for your reply. I agree that I should observe the same results.
I observed these results by repeating these tests, so I was a bit worried.
I will run longer simulations and will post the outcome here.

Mandar Kulkarni