[gmx-users] umbrella sampling

[André Farias de Moura]

I never did it myself, but generally speaking you don' t expect that there'
s a lot of room inside a crystal for any molecule to diffuse there,
especially when it comes to cross something like a crystallographic plane,
meaning that huge energy barriers should be there, leading to NaN and other
weird numerical issues.

in principle, larger force constants might help you to sample high energy
barrier, but it will be most likely a trial and error procedure until you
fine-tune the forces along the profile.

best

Andre

On Tue, Jul 11, 2017 at 1:09 PM, Ben Tam <btam125 at hotmail.co.uk> wrote:

> Dear All,
>
> I am doing umbrella sampling for a water molecules moving inside a crystal
> structure, however I am running into a problem on the output file with
> profile.xvg all value showing “nan”. This error has occurred when I reduce
> the slide width from 0.2 nm to 0.1 nm. I have used the exact pull code for
> 0.2 and 0.1, however only 0.2 has given me meaningful profile. The pull
> code I used is the following:
>
> ;Pull code
> pull = yes
> pull_ngroups = 2
> pull_ncoords = 1
> pull_group1_name = MOL
> pull_group2_name = SOL
> pull_coord1_type = umbrella ; harmonic biasing force
> pull_coord1_geometry = distance ; simple distance increase
> ;pull_coord1_vec = 0 0 1
> pull_coord1_groups = 1 2
> pull_coord1_dim = N N Y
> pull_coord1_rate = 0.01 ; 0.01 nm per ps = 10 nm per ns
> pull_coord1_k = 10 ; kJ mol^-1 nm^-2
> pull_coord1_start = yes ; define initial COM distance > 0
>
> and the time step I used is 500 ps, but I have also tried 100 and 50 ps. I
> have tried increasing the k value to 50 and 100 with the same results.
>
> The histogram shows many overlap, however when I use k value 100, it is
> only giving me individual single peak. Therefore can someone enlighten on
> what is going on?
>
> Best regards,
>
> Ben
>
>
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-- 
_____________

Prof. Dr. André Farias de Moura
Department of Chemistry
Federal University of São Carlos
São Carlos - Brazil
phone: +55-16-3351-8090