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

[Yang Ye]

On 7/10/2007 2:53 PM, 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 ). So I just want to know how the
> developers of gmx or gromos force field get Lennard-Jones parameters for
> Na+.
>   
They are never simple and tends to be problematic. The recent concern 
over abnormal clustering of excess K/Na with Cl in AMBER force field has 
highlighted the need of better parameterization of ions. A short 
description of this problem is on page 5 of

Auffinger, P., Hashem, Y. Nucleic Acid Solvation: from outside to 
insight, Current Opinion in Structural Biology, 2007 17:1-9

Back to your problem, you may dig the literature about the root of these 
two set of parameters.
> 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
>
>
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