[gmx-users] Re: Wierd results from Umbrella, sampling (Justin A. Lemkul)

Thomas Schlesier schlesi at uni-mainz.de
Tue May 22 16:36:47 CEST 2012

Think it would be best to show the .mdp file, else we can only guess 
what the parameters are.
 From the histogram it looks like that the force constant of the 
restraining potential is too low, since the histograms are really wide, 
but pull_k1=1000 is a 'normal' value.
On thing which confueses me is you said that fluctuations from g_dist 
are about 0.4nm, but in the histogram i looks like the distance 
fluctuates about 1nm or even more. for this be sure, that you compare 
the right distances -> if you pull only in z-direction, the only look 
into the z-direction from the output from g_dist. Since the x, and 
y-directions would be unaffected by the umbrella potential.

for the error message with the constraints:
what happens if you run the system with constraints but without the 

for pull_k1>0 and pull_rate1=0:
if you're pulling with an umbrella potential pull_k1 defines the force 
constant of the potential (hook'sches law). Let's assume you put a 
spring to your molecule
with pull_rate1=0, it means that the other end of the spring doesn't 
move, and the spring restrains the position of the molecule (molecule 
cn't diffuse away). in umbrella sampling you want to restrian of 
molecule (or part of it) relative to another one / part -> so pull_rate1=0
with pull_rate1>0, it's like you pull the spring away for the molecule, 
spring gets strechted -> wants to relax -> molecule follows spring (like 
that what happens in afm pulling)

Am 22.05.2012 15:40, schrieb gmx-users-request at gromacs.org:
> Hi Justin, As for the maximum of 0.4 nm fluctuation of my pulled
> protein, I used g_dist to calculate the distance between my pulled
> protein and stable part in a window, where the distance is treated as
> the fluctuation of the protein along z-axis. Well, from pullx.xvg, the
> position moved a lot (3nm for instance.) As for the windows simulation,
> I didn't apply constraint but only the internal constraint in the itp
> file. I still don't understand why it have to do constraint? why not
> give a fully freedom to run the simulation? In the md_umbrella.mdp, I
> set pull_k1=1000; pull_rate1= 0.0, but apparently, I am confused with
> pull_k1>0 combined with pull_rate1=0. In my mdp, i set "none" to LINCS,
> because if I use "all-bonds", an error of "1099 constraints but degrees
> of freedom is only1074" occurs. Actually, there is no any window with a
> designed distance. Here I attach the histo.xvg, profile.xvg. Thank you
> with regards, Jiangfeng. On 5/22/12 9:36 AM, Du Jiangfeng (BIOCH) wrote:
>> >  Dear Justin,
>> >
>> >  Based on your questions to my simulation, I posted here yesterday hopefully
>> >  it was the correct way to reply in this forum.
>> >
> You've still replied to the entire digest message (which I've cut out); please
> make sure to keep replies free of superfluous posts in the future.  The archive
> is already pretty hopeless, but let's not make it worse:)
>> >  In this morning I got a list of new windows of umbrella sampling, the overlap
>> >  is sufficient enough, but I saw another problem:  In the histogram figure,
>> >  the base of peak covers the distance of 2 nm instead of 0.2 nm., that's
>> >  horrible! However, when I checked back to the simulation results of each
>> >  window, the fluctuation of my pulled protein is only 0.4nm in maximum. So the
>> >  base of peak shouldn't cover such long distance, right?
>> >
> If the peaks aren't corresponding to the desired restraint distances, then there
> are several potential problems:
> 1. Your restraints aren't set up the way you think they are (check grompp output
> and .tpr file contents to be sure)
> 2. Your restraints are ineffectual (in which case you may need to revisit the
> force constant)
> I can't determine from your description what's going on.  What do you mean by a
> maximum of 0.4 nm fluctuation?  In what quantity?  What do the contents of
> pullx.xvg show you for the problematic window, and for that matter, the others?
>    Are any of them producing the desired restraint distances?
> Again I will ask you to share an image of the histogram and PMF profile; these
> would be very helpful to see.
> -Justin
> --
> ========================================

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