# [gmx-users] How to write pull code in umbrella sampling

Qing Lv lvqingjiejie at 163.com
Sat Apr 8 07:46:50 CEST 2017

```Thank you, Justin.

My goal is to investigate the conformation transition of a 30aa loop between an unstructured loop and an alpha-helix. I found a transition pathway on free energy landscape and selected a number of frames along the pathway for umbrella sampling. So, should I define C-alpha of this 30aa loop as pull-group1 and C-alpha of the remaining part as pull-group2 ? Would the pull geometry still be "distance"? But this will just restrain the distance between COM of the 2 groups to the initial distance, but cannot restrain the conformation of the loop.
Or need I divide the loop into many small segments (pull-groups) and define the pull coordinates separately? This seems horrible...

Thanks,
Qing

At 2017-04-08 09:06:46, "Justin Lemkul" <jalemkul at vt.edu> wrote:
>
>
>On 4/7/17 8:56 PM, Qing Lv wrote:
>> Dear Colleagues,
>>
>>
>> I am trying to do an umbrella sampling using a series of conformations extracted from several unbiased MD trajectories. But I am not sure about the pull code in .mdp file.
>> Below is the pull code from the tutorial of Dr. Justin Lemkul :
>> define      = -DPOSRES_B ; (Why ??)
>
>Did you read the paper I linked from the tutorial, which explains why this is
>done for this very specific case?
>
>> ; Pull code
>> pull                    = yes
>> pull_ngroups            = 2
>> pull_ncoords            = 1
>> pull_group1_name        = Chain_B
>> pull_group2_name        = Chain_A
>> pull_coord1_type        = umbrella      ; harmonic biasing force
>> pull_coord1_geometry    = distance      ; simple distance increase
>> pull_coord1_groups      = 1 2
>> pull_coord1_dim         = N N Y
>> pull_coord1_rate        = 0.0
>> pull_coord1_k           = 1000          ; kJ mol^-1 nm^-2
>> pull_coord1_start       = yes           ; define initial COM distance > 0
>>
>>
>>
>> Conformations in Justin's tutorial are from steered MD. However, my conformations are not from SMD, and I have not done SMD on this system. So, how to define pull_group1 and pull_group2? And pull_coord1_geometry still set to "distance" ?
>
>The pull groups don't depend on where the configurations came from.  They define
>the reaction coordinate.  You can use whatever pull geometry makes sense; in the
>tutorial it is a simple linear reaction coordinate directly along the z-axis.
>Other applications may require different definitions.
>
>> Also, need I fix any part of protein in umbrella sampling (with " -DPOSRES ") ? Why?
>
>Generally, no, but you haven't given any useful detail about what your
>simulation system is.
>
>-Justin
>
>--
>==================================================
>
>Justin A. Lemkul, Ph.D.
>Ruth L. Kirschstein NRSA Postdoctoral Fellow
>
>Department of Pharmaceutical Sciences
>School of Pharmacy
>Health Sciences Facility II, Room 629
>University of Maryland, Baltimore
>20 Penn St.
>Baltimore, MD 21201
>
>jalemkul at outerbanks.umaryland.edu | (410) 706-7441
>http://mackerell.umaryland.edu/~jalemkul
>
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