[gmx-users] force field parameters for hydronium

[Mark Abraham]

Bert wrote:
> Hi gmx-users,
>  
> I wonder if I can add H3O+ (Eigen) and OH- combined with SPC/E water 
> model into my system. I have not found any available force field 
> parameters for them in gmx yet, does anyone has itps and gros file for 
> modeling them? BTW, I do not care about the PH value, and  I only treat 
> them as simple ions. Thanks for your suggestions.  

I'm not aware of one. Trying to model hydronium is a pretty flawed thing 
to do, since the hydrogens in real water are labile - so the molecule 
you'd label as "hydronium" moves much faster than any atoms do. Other 
ions don't do this, of course. Since MD water hydrogens are not labile, 
the hydronium model wouldn't be one of anything physically recognizable.

You also don't have anywhere near enough waters in a typical simulation 
system to get anything like a normal pH, even if there was a suitable 
hydronium-hydroxide model.

Mark