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

Adding Fix

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 the fix

#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)