From: Massimiliano Porrini (
Date: Fri Feb 28 2014 - 05:18:51 CST

Hi all,

I am optimising atomic charges (from water-interaction profile) of
a cationic (+1) molecule with ffTK of VMD and I quote below
what is stated at Pag. 2760, 1st column, of the related paper
(C.G. Mayne, J. Saam, K. Schulten, E. Tajkhorshid, J.C. Gumbart
J. Comput. Chem. 2013, DOI: 10.1002/jcc.23422.):

*To better approximate the bulk-phase, for polar target compounds
theQM-optimized distances are shifted by an offset of -0.2 Angs andthe
interaction energies are scaled by 1.16 (for neutral molecules
only),although both parameters can be adjusted by the user.*

Therefore, since the molecule I am parametrising is not neutral but a
I do not have to scale the interaction energies (that is Scale = 1.0).

What is not clear to me, from the paper statement, is whether
the QM-optimised distances do not need to be
offset by -0.2 Angs (in case of non-neutral molecules) either.

Thanks in advance.


Dr Massimiliano Porrini
Institut Européen de Chimie et Biologie (IECB)
CNRS UMR 5248 Chimie et Biologie des Membranes et des Nano-objets (CBMN)
2, rue Robert Escarpit
33607 Pessac Cedex