[gmx-users] question about positive potential
Mark.Abraham at anu.edu.au
Fri May 22 16:14:30 CEST 2009
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
See http://wiki.gromacs.org/index.php/Exotic_Species and
> system potential,
> kJ/mol No. of atoms
> PE+ & PE- 7.89481e+03(+-347)
> 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.
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