[gmx-users] question about positive potential

[Mark Abraham]

Baofu Qiao wrote:
> Hi Mark,
> 
> There are two polyelectrolytes in my systems: poly(sodium
> 4-styrenesulfonate) and poly(diallyl dimethyl ammonium chloride). And
> Actually sodium and chloride are absent, only the polycation and the
> polyanion. I didn't find suitable force field for sulfonate, thus I used
> the reported force field in Ionic liquid for sulfonate (which is based
> on opls-aa) , and opls-aa force field parameters for the other atoms.
> 
> When I add some water into the polyelectrolytes complex systems, 
> potential becomes negative.  See the comparison below, (both at 400K)

There's no reason to suppose parameters suitable for a sulfonate group 
in some ionic liquid are transferable to the same group in aqueous 
solution. You could easily be observing the proof of that. Parameters 
aren't magic - they may have been developed to reproduce certain 
observables under certain conditions. The further you move from those 
conditions, the less likely the parameters are to be valid without 
further testing.

See http://wiki.gromacs.org/index.php/Exotic_Species and 
http://wiki.gromacs.org/index.php/Parameterization

>                   system                              potential, 
> kJ/mol            No. of atoms
>                   PE+ & PE-                  7.89481e+03(+-347) 
>           4080
>                 PE+ & PE-+water:       -3.04580e+04(+-537)           7080
> Any other suggestions?
> 
> I wonder that is the size of the system related to this problem? and
> when the potential is positive, why doesn't the system becomes expanded
> (Does positive potential have some physics)?

The size of your potential results from increases in the amount of PE in 
your bonded interactions - presumably getting over-stretched from the 
high temperature, or unsuitable parameterization or both. Further 
expansion of the system wouldn't relieve this, except inasmuch as the 
KE/PE partitioning might change.

In an electrostatic system lacking bonded interactions, positive 
potential reflects that a lower energy state could be achieved by 
expanding the system.

Mark