[gmx-users] Gromacs vs. Ab initio charges

[quantix1 at gmx.de]

Hi Vojtech,

force field charges are usually taken from quantum calculations (normally ab
initio).
The electrostatic potential  on a grid of points around the molecule is
calculated
from the wavefunction. The atomic point charges are then fitted to represent
the ESP
as good as possible. There are several methods available to do that. Look
for RESP 
or CHELPG in the literature. So basically, the FF charges are ab initio
charges, although
you might know that the atomic charge is not a quantum machanical
observable. This
means there is no way to assign the electrons of the molecule to specific
nuclei uniquely. 
The charges based on a fit of the ESP just seem to work best with force
fields.

Best regards,
Uwe


> Good Morning
> As Gromacs charges are usually lower than those calculated
> ab initio by for example Kollman/Singh method is there any
> way how to make them compatible with Gromacs by for
> example multiplication of them by some factor ?
>  
> Thanks
> Vojtech Spiwok
> ICT Prague
> 
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