Dear Gromacs users,
I have encountered a strange problem when trying to set up umbrella sampling
simulations using gromacs 4.5.3.
My system contains two groups with a COM distance of 1.95 nm (all distances
measured by g_dist).
Trying to set up the first US window, I use the following pull code, together
with typical mdp parameters from the Gromacs US tutorial:
; pull options
pull = umbrella
pull_geometry = distance
pull_dim = Y Y Y
pull_start = yes
pull_init1 = 0.00
pull_nstxout = 1
pull_nstfout = 0
pull_group0 = Protein
pull_group1 = OHM
pull_rate1 = 0.0
pull_k1 = 10000
this results in a simulation in which the group distance remains close to
1.95nm, as expected.
However, when I make the following two changes in my input file:
pull_start = no
pull_init1 = 1.95
which should (?) amount to an equivalent setup, a very different trajectory
results in which the COM distance quickly increases to 2.7 nm and then appears
to be restrained there. (Visualization confirms, in the first case, the groups
remain in their starting conformation, in the second one, they are pushed
appart)
Interestingly, the grompp output contains the following lines:
Pull group natoms pbc atom distance at start reference at t=0
0 994 497
1 2 46347 1.224 1.950
in this case. Apparently, grompp (and mdrun thereafter) calculates the group
COM distance differently from g_dist! I think this is not a PBC issue, every
atomic distance within both groups is smaller than half the box size and the
pbcatoms are close together. However, when I set pbcatom0 to various atom
numbers, different 'distance at start' values are obtained, but never the
correct COM distance. The two groups do not have overlapping atoms. I am sure I
used the same group indexes for pulling and distance measurements.
This behaviour is so visibly wrong that I cannot believe this is a bug and
rather think I am doing something incorrect. A search of the list revealed a
somewhat similar report by Gavin Melaugh in 2011 which did not resolve the
issue.
Any ideas on what might be the problem here?
I am willing to send around files to reproduce the problem of course.
Thomas
Dr. Thomas Steinbrecher
Institut für Physikalische Chemie, KIT
Kaiserstr. 12, 76131 Karlsruhe--
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