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

[Thomas Schlesier]

Hi,
I never used the pull-code with GMX 4.5.x or 4.6.x,but I would assume 
that it'sthe same as in 4.0.x:
'Umbrella': Due to the harmonic potential the force and the distance 
between the two particles will fluctuate.
'Constraint': Only the force will fluctuate. The (relative) distance 
between the two particles should either stay constant (if v=0) or 
increase linearly (v.ne.0). But still the position of both groups can 
fluctuate, but only in such a way that the constraint isn't violated.
Greetings
Thomas


Am 30.11.2013 15:41, schrieb 
gromacs.org_gmx-users-request at maillist.sys.kth.se:
> 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