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Test system #1: box with two chloride ions in vacuum
(1a) run standard gromacs gromacs5.1.2 replica exchage with two replicas (one at 300 K and the other at 300.01 K) -- this seemed to work as expected.
(1b) run gromacs5.1.2 patched with plumed2.2.2-hrex with two replicas, both at 300 K and both with lambda=0 -- as expected, all energies are zero and all exchange attempts have P=1.0
(1c) as #1b except with both lambdas=1 -- as expected, all exchange attempts have P=1.0
(1d) as #1b except with both lambdas=0.5 -- as expected, all exchange attempts have P=1.0
(1e) as #1b except with lambdas at 1.0 and 0.001 -- as expected, exchange attempts have P=0.0. I then use gmx traj to get the full precision coordinates (saved using xtc-precision=1000000) and evaluate the energies, which match incredibly well (this is a double precision run):
me: 81.5086 plumed: 8.151e+01 from distances: 0.914412 and 0.394249 at time 0.002
me: 80.4921 plumed: 8.049e+01 from distances: 0.912717 and 0.396451 at time 0.006
me: 79.4887 plumed: 7.949e+01 from distances: 0.911223 and 0.398699 at time 0.01
me: 78.4993 plumed: 7.850e+01 from distances: 0.909893 and 0.400985 at time 0.014
me: 77.5297 plumed: 7.753e+01 from distances: 0.908568 and 0.403266 at time 0.018
…
me: 56.8677 plumed: 5.687e+01 from distances: 0.9371 and 0.478046 at time 0.122
me: 56.3657 plumed: 5.637e+01 from distances: 0.940528 and 0.481002 at time 0.126
me: 55.9071 plumed: 5.591e+01 from distances: 0.944196 and 0.483872 at time 0.13
…
However, when one of the replicas experiences particle separation that go outside the cutoffs, plumed doesn't seem to understand that. Below, you'll see that at time >=0.186 ps the energies diverge.
…
me: 51.5971 plumed: 5.160e+01 from distances: 0.983643 and 0.513695 at time 0.17
me: 51.2172 plumed: 5.122e+01 from distances: 0.988269 and 0.51678 at time 0.174
me: 50.8628 plumed: 5.086e+01 from distances: 0.993142 and 0.519835 at time 0.178
me: 50.5459 plumed: 5.055e+01 from distances: 0.998129 and 0.52276 at time 0.182
me: 105.687 plumed: 5.005e+01 from distances: 1.00308 and 0.525746 at time 0.186
me: 105.067 plumed: 4.943e+01 from distances: 1.00781 and 0.528857 at time 0.19
me: 104.428 plumed: 4.879e+01 from distances: 1.01259 and 0.532105 at time 0.194
me: 103.777 plumed: 4.814e+01 from distances: 1.01738 and 0.535455 at time 0.198
It gets stranger in that when I re-evaluate the energies assuming that the cutoff was much larger (same simulation data, just not truncating at 1 nm), then I get this:
…
me: 50.5459 plumed: 5.055e+01 from distances: 0.998129 and 0.52276 at time 0.182
me: 50.2188 plumed: 5.005e+01 from distances: 1.00308 and 0.525746 at time 0.186
me: 49.859 plumed: 4.943e+01 from distances: 1.00781 and 0.528857 at time 0.19
me: 49.4801 plumed: 4.879e+01 from distances: 1.01259 and 0.532105 at time 0.194
me: 49.0873 plumed: 4.814e+01 from distances: 1.01738 and 0.535455 at time 0.198
And you will note that the match is not really so good after the point where the cutoff was crossed, though the approximate value is now correct.
One can fix this entirely by adding the following lines to the .mdp file:
coulomb-modifier=none
vdw-modifier=none
e.g. (from a different simulation since the simulation had to be conducted with new .mdp options):
...
me: 192.512 plumed: 1.925e+02 from distances: 1.25717 and 0.32504 at time 0.386
me: 191.909 plumed: 1.919e+02 from distances: 1.26005 and 0.325476 at time 0.39
me: 191.256 plumed: 1.913e+02 from distances: 1.26297 and 0.325955 at time 0.394
me: 190.491 plumed: 1.905e+02 from distances: 1.26591 and 0.326524 at time 0.398
I'm not sure exactly what's going on here. Anyway, I think there is some issue when using the default Potential-shift-Verlet modifiers.
I'm going to continue testing with some more systems that are a bit more realistic and will report back, but if anybody can make any sense of the fact that problems arise with coulomb-modifier=Potential-shift-Verlet and vdw-modifier=Potential-shift-Verlet (default options for gromacs 5.1.2), then I am very interested to hear about it.
Mdp file:
cutoff-scheme = verlet
constraints = all-bonds
lincs-iter = 1
lincs-order = 6
constraint_algorithm = lincs
integrator = sd
dt = 0.002
tinit = 0
nsteps = 200
nstcomm = 1
nstxout = 1
nstvout = 1000000000
nstfout = 1000000000
nstxtcout = 1
nstenergy = 1
nstlist = 1
nstlog=0 ; reduce log file size
ns_type = grid
rlist = 1.0
rvdw = 1.0
rcoulomb = 1.0
coulombtype = cut-off
tc_grps = System
tau_t = 1.0
ld_seed = -1
ref_t = 300
gen_temp = 300
gen_vel = yes
unconstrained_start = no
gen_seed = -1
Pcoupl = no
nstcalcenergy=1
xtc-precision=1000000
coulomb-modifier=none ;; only included for last test
vdw-modifier=none ;; only included for last test
####
Thank you,
Chris.
Replica exchange statistics
Repl 49999 attempts, 25000 odd, 24999 even
Repl average probabilities:
Repl 0 1 2 3 4 5 6 7
Repl .60 .62 .64 .65 .66 .67 .68
Repl number of exchanges:
Repl 0 1 2 3 4 5 6 7
Repl 15023 15662 15943 16274 16519 16734 17047
Repl average number of exchanges:
Repl 0 1 2 3 4 5 6 7
Repl .60 .63 .64 .65 .66 .67 .68
Replica exchange statistics
Repl 9999 attempts, 5000 odd, 4999 even
Repl average probabilities:
Repl 0 1 2 3 4 5 6 7
Repl 1.0 .56 1.0 .58 1.0 .62 1.0
Repl number of exchanges:
Repl 0 1 2 3 4 5 6 7
Repl 5000 2817 5000 2925 4999 3057 5000
Repl average number of exchanges:
Repl 0 1 2 3 4 5 6 7
Repl 1.0 .56 1.0 .59 1.0 .61 1.0
I am not sure to know where to look for to give more information. Please let me know what I can do to help.
Nicolas
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Hi,
In both cases I have used the same input files (except the pull-code part that needs to be changed between Gromacs4.6.7 and Gromacs5.1.2). In particular, in the .mdp "nstlist = 20" and in the command line "-replex 100". So the answer to your question is yes.
Nicolas
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Replica exchange statistics
Repl 4999 attempts, 2500 odd, 2499 even
Repl average probabilities:
Repl 0 1 2 3 4 5 6 7
Repl .60 .60 .62 .63 .65 .66 .67
Repl number of exchanges:
Repl 0 1 2 3 4 5 6 7
Repl 1534 1467 1591 1531 1634 1683 1697
Repl average number of exchanges:
Repl 0 1 2 3 4 5 6 7
Repl .61 .59 .64 .61 .65 .67 .68
I did a small benchmark. Everything is the same (system, starting points, input files, etc...). I am using 8 replicas with HREX+Umbrella Sampling (the pull code has changed between the two versions of Gromacs, which is very likely the reason for the change in performances). Nodes have 12 cores, I used either 8 cores in one node or used 2 nodes (24 cores)
ns/day hours/ns
Gromacs4.6.7/Plumed2.1, 8cores 2.462 9.749
Gromacs4.6.7/Plumed2.1, 24cores 5.180 4.633
Gromacs5.1.2/Plumed2.2, 8cores 3.165 7.582
Gromacs5.1.2/Plumed2.2, 24cores 6.763 3.549
Nicolas
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