[gmx-users] How to do an osmotic pressure calculation in Gromacs 5.1.4.

[Justin Lemkul]

On 4/25/17 10:29 AM, Miner, Jacob Carlson wrote:
> Hello,
>
>
> I am trying to set up a cube (LxLxL) of 2000 TIP3P waters and 72 urea molecules at 300 K with an adjacent cube (LxLxL) containing only water.
>
> The aim with this system is to place a semipermeable membrane between along the dimension of the two LxLxL boxes (now a box of LxLx2L) so that a cosolvent particle that attempts to cross from one box to the other (Z displacement < 0 or Z-disp > L) will be pushed back by a flat-bottom potential. The water, meanwhile, should be allowed to transit through the entire 2L Z-dimension without any opposing force. Periodic boundary conditions will be present along the X and Y dimensions as well.
>
> This was accomplished in a previous version of gromacs assembled by my colleagues in gromacs 4.0.7 and 4.5.5 who actually added their own pulling potential to the code.
>
>
> As the formatting has changed, and now a flat-bottom potential is included in the newer versions of Gromacs, I wanted to know if anyone can explain how the code could be set up to allow this kind of force measurement to be conducted in Gromacs v5.1.4?
>

You can do it in 5.1.4 using a series of steps employing flat-bottom 
[position_restraints] but it is rather complex.  I suggest using the pull code 
of version 2016.3:

http://manual.gromacs.org/documentation/2016.3/user-guide/mdp-options.html#com-pulling

You'll set two restraints, one with type "flat-bottom" and the other with 
"flat-bottom-high" and specify the atoms to which the restraint is applied along 
the z-axis.  That should be all there is to it (I have not tried this method but 
it should work; I have been doing osmotic pressure calculations in NAMD where it 
is also very easy).

-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

==================================================