# DSF Charge Pair potential¶

## Overview¶

Pairwise force shifted damped Coulomb as defined by Christopher J. Fennel and J. Daniel Gezelter J. Chem. Phys (124), 234104 2006 Eqn 19.

$\begin{split}F(r_{ij}) = \left[\begin{array}{cc} q_i q_j \left(\frac{{\rm erf}(\alpha r)}{r^2} +\frac{2\alpha}{\sqrt{\pi}} \frac{{\rm exp}(-\alpha^2 r^2 )}{r} -\frac{{\rm erf}(\alpha r_{\rm cut})}{r_{\rm cut}^2}+\frac{2\alpha}{\sqrt{\pi}}\frac{{\rm exp}(-\alpha^2 r_{\rm cut}^2)}{r_{\rm cut}}\right),& r<r_{\rm cut}\\ 0, & r\geq r_{\rm cut} \end{array}\right.\end{split}$

where $$q_i, q_j$$ are charges of particles $$i,j$$, $$r_{ij}$$ is the distance between particles $$i,j$$, $$\alpha$$ is convergence parameter, and $$r_{\rm cut}$$ is cutoff distance.

parameters of potential can be defined directly within the python input script, or read from a restart file.

## Python Member Functions¶

FixChargePairDSF(state=..., handle=...,group_handle=...)


Arguments

state
state object to add the fix.
handle
A name for the fix.
group_handle
A group name to apply potential.

Setting parameters from within the Python environment is done with setParameters.

setParameters(alpha=...,r_cut=...)


Arguments

alpha

name of parameter to set. Can be eps, sig, rCut.

rCut parameter has a default value equal to state.rCut.

r_cut
Cutoff radius for charge calculations

Default parameter values are $$\alpha=0.25$$, $$r_{\rm cut}=9.0$$, assuming $$\sigma_{\rm LJ}=1.0$$

## Examples¶

#adding charge fix
charge=FixChargePairDSF(state, "charge","all")


Setting parameters in python

charge.setParameters(alpha=0.25,r_cut=9.0);


Activating the fix

#Activate fix
state.activateFix(charge)