[gmx-users] umbrella versus constraint in the pulling code

[Dr. Vitaly Chaban]

Could anyone please explain the difference between how "umbrella" and
"constaint" options in the pulling code work. Based on the manual, I
expect that the only difference
is that umbrella used harmonic potential to bind pulled group to the
reference group (point), while "constraint" uses shake algorithm to
rigidly bind the groups. However, the simulations shows that this is
not true. With all other options being the same, umbrella provides
expected pulling, while constraint exhibits rather fluctuations of the
pulled group nearby starting position. What do I miss?

Dr. Vitaly V. Chaban

output from the "constraint" simulation with moving reference point
(i.e. pull_geometry = position)
z-coordinate
 6.38675
 5.97254
 5.88948
 5.86992
 5.72523
 5.59002
 5.84313
 5.83232
 5.88722
 5.83665
 6.00198
 5.96873
 5.57108
 5.35213
 5.47916
 5.46968
 5.40241
 5.41725
 5.4732
 5.45464
 5.61582
 5.66366
 5.8005
 5.9086
 6.10943
 6.11296
 6.21684
 5.95532
 5.89226
 6.00983