[gmx-users] Re: The Lennard-Jones parameter of Na+? (Mark Abraham)

[Mark Abraham]

Hu Zhongqiao wrote:
>  Dear Mark
> 
> Thanks for your reply.
> 
> In more detail, I used ffg43a1 force field. In the file ffG43a1nb.itp,
> one can find the Lennard-Jones parameters for Na+ as follows:
> 
> C6=7.2063121e-05; and C12= 2.1025e-08
> 
>>From these 2 values one can get epsion(Na+)=0.0617 kJ/mol. 
> 
> Because the force field parameters for ions is relatively simple, the
> main interaction of Na+ with other particles is electrostatic and van
> der Waal's forces. And I also believe that the combination rule makes
> not big difference for different force fields in this case. It is
> strange epsion(Na+) = 0.0617 kJ/mol in ffg43a1 is very different from
> some references (for example, in the paper shown in my last email, this
> value is epsilon(Na+) = 0.42 kJ/mol ). 

This is not strange at all. These force fields are probably optimized to 
achieve different targets under different simulation conditions on a 
different test set. There is no wonder that they are different, and 
there's no particular expectation of correlation with any property of 
real Na+. Ensemble average properties of systems with solvated Na+ 
should be reasonable, however.

> So I just want to know how the
> developers of gmx or gromos force field get Lennard-Jones parameters for
> Na+.

You should find the original literature that describes them and read it. 
That's why the authors wrote it :-)

Mark