[gmx-users] forcefield for free (neutral) amino acids

[FyD]

Dear chun feng,

> I am not a user of GROMACS, but I hope someone who's working on   
> protein dynamics can do me a favor here. I am desperately searching   
> for a forcefield for free and neutral amino acids. But it seems that  
>  the forcefields such as amber and opls only parametrizes amino  
> acids  in peptides (without OH and H) and terminal amino acids on  
> peptides.

Yes

> I know there has to be some forcefield already developed for popular  
>  molecules like amino acids, anyone please point me the way! Thanks  
> a  million!

You could derive your own force field for such amino-acids. You can  
derive RESP and ESP charges for any molecule or molecular fragment  
containing chemical elements up to Bromine using the R.E.D. tools. See  
http://q4md-forcefieldtools.org/

regards, Francois