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

[Hu Zhongqiao]

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


Zhongqiao

> Dear all,
>  
> I find the Lennard-Jones parameter epsilon of Na+ in Gmx is very 
> different from some references.
>  
> In gmx, I calculated sigma(Na+)=0.258nm, epsilon(Na+)=0.0617 kJ/mol
>  
> But many researchers cited a paper by Dang LX [J. Am. Chem. Soc.,
1995, 
> 117, 6954-6960] and used parameters as follows sigma(Na+)=0.258nm, 
> epsilon(Na+)=0.42 kJ/mol.
>  
> I want to ask where parameter of Na+ in gromos force field comes from?

> Is it reasonable?

In isolation, no force field parameter has any meaning. See 
http://wiki.gromacs.org/index.php/Force_Fields#Usage

Mark