[gmx-users] Why partial charge on chlorine atom from GAFF is +ve?

[Lakshman Ji Verma]

Hi everyone,

I am looking for an Amber based force field for chloroform as a solvent to
use with Gromacs. I adopted GAFF ff from http://virtualchemistry.org.

The partial charge on chlorine and carbon which are +ve and -ve,
receptively, are a bit confusing. Since chlorine is more electronegative
than the carbon, it should have -ve charge and carbon should be +vely
charged which was the case with OPLS based ff I used earlier, which I
generated with LigParGen server.

I am wondering why is this the case with GAFF and is there any logical
explanation behind it?

PS: I need Amber based ff for the compatibility with ff for other molecules.

Any kind of help would be appreciated.

Thank you
Lakshman