setting the polarizabilities for the Drude force field

From: Frank Haverkort (
Date: Thu Aug 01 2013 - 06:48:59 CDT

I'm preparing to use the Drude FF for my simulations. Because I don't
have access to the CHARMM program (which can generate Drude PSF files),
I want to write a script which makes the Drude PSF file using as input
the corresponding non-Drude PSF (which VMD can make for me). Now it
seems that I need to supply reduntant parameters for the Drude
polarizabilities, and I'm wondering if my understanding of this is correct.

In the PSF file, the drude charge q_D and the polarizability alpha are
specified. On the other hand, the force constant for the drude particle
k_D is specified in the force field parameter file. But since these
quantities are related via alpha = q_D^2 / k_D, why is it not sufficient
to define only two of them in the input files? Now it seems that I
manually have to calculate the force constant k_D from a given alpha and

Furthermore, I found a contradictory statement in the Drude
documentation of the NAMD website (
The force constant kD, which is assumed to be the same for all atoms
without any loss of generality in the value of alphaal
This statement seems to contradict the statement I found on this mailing
list, that the Drude force constants are specified in the force field
parameter file. In that file, I have observed that you can indeed
seperately set the bond force constant for each combination of two atom
types. Is this an error on the side of the documentation, or am I
missing something?

Hope any of you can clarify this!


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