[gmx-users] Turning off electrostatics between groups using custom tables?

[Mark Abraham]

On 1/09/2012 1:14 AM, Smitty wrote:
> Hi,
> I've been trying to turn off the electrostatic interaction between two
> groups (an ion-group and protein) by following the tutorial on custom table
> generation, but have seemed to have hit a road block.
>
> Looking at the structure of the sample 6-12 table provided in the topology
> directory, I constructed my table.xvg for interactions between the two
> groups to be the following:
>
> #r coulomb coulomb-force 1/r^6 6/ r^7 1/12 12/r^13
> 0    0           0                   0         0       0       0
> .
> 0.04 0          0                 -2.4414062500e+08 -3.6621093750e+10
> 5.9604644775e+16 1.7881393433e+19
>
> and so on.  Noting that the second and third columns are defined as zero for
> all steps, while the columns related to LJ are left unchanged.
>
> While for the protein-protein or ion-group ion-group interactions I provide
> just the standard 6-12 table. The problem comes that when I run using my
> modified table, the system will run but gradually explodes. I note that when
> I look back at my output from grompp I find a warning concerning eplison and
> sigma being used instead of C6 and C12,

Ignoring warnings is a great way to waste time, and if you can't provide 
specific details of what it was, you're unlikely to get specific help.

>   but if I swap my custom table for
> the sample 6-12 table (keeping the first two columns not zero) the system
> doesn't explode and has dynamics similar to previous simulations making use
> the built-in table generation. Any ideas on where I am going wrong?

Can't tell. What's the "ion group"? Does it even get close to Protein?

Mark

>   
>
> Here are the other specs for the simulations:
> Gromacs 4.0.5, OPLS/AA, berendsen pressure/temperature coupling: tau_p =0.5,
> tau_t=0.1, simulation temperature 283.
>
> Any help is much appreciated!
> -Smitty
>
>
>
>
> --
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