# Force shifted LJ Pair potential¶

## Overview¶

Define a pair potential energy function corresponding to the modified Lennard-Jones potential

$\begin{split}V(r_{ij}) = \left[\begin{array}{cc} 4 \varepsilon \left( \left(\frac{\sigma}{r_{ij}}\right)^{12} - \left(\frac{\sigma}{r_{ij}}\right)^{6}\right)-F_{\rm LJ}(r_{\rm cut})r_{ij},& r<r_{\rm cut}\\ 0, & r\geq r_{\rm cut} \end{array}\right.\end{split}$

where $$r_{ij}$$ is the distance between particles $$i,j$$ $$\varepsilon, \sigma$$ are Lennard-Jones potential parameters, and that must be set to define the interaction between atoms, $$r_{\rm cut}$$ is cutoff distance, and $$F_{\rm LJ}(r_{ij})$$ is the original Lennard-Jones force:

$F_{\rm LJ}(r) =\frac{\partial V_{\rm LJ}(r)}{\partial r} (r).$

This modified Lennard-Jones potential has force equal to 0 at cutoff distance.

parameters of potential can be defined directly within the python input script, read from a LAMMPS data file (using the LAMMPS reader utility), read from a NAMD input file (using the NAMD reader utility) or read from a restart file.

## Python Member Functions¶

Adding Fix

FixLJCutFS(state=..., handle=...)


Arguments

state
state object to add the fix
handle
A name for the fix.

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

setParameter(param=...,handleA=...,handleB=...,val=...,)


Arguments

param

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

rCut parameter has a default value equal to state.rCut. All other parameters have to be set manually.

handleA, handleB
a pair of type names to set parameters
val
value of the parameter.

It is also possible to get value of the parameters within the Python environment with getParameter.

val = getParameter(param=...,handleA=...,handleB=...)


Arguments

param
name of parameter to set. Can be eps, sig, rCut
handleA, handleB
pair of type names to set parameters for
val
value of the parameter

## Examples¶

Adding the fix

#adding force-shifted Lennard-Jones potential
ljcut = FixLJCutFS(state, handle='ljcut')


Setting parameters in python

ljcut.setParameter(param='eps', handleA='A', handleB='B', val=1.0)
ljcut.setParameter(param='sig', handleA='B', handleB='B', val=1.0)


Setting same parameters for all types in python

ljsig=1.0
ljeps=1.0
#list of all types
types=['A','B','C','S','P','N']
for A in types:
for B in types:
ljcut.setParameter(param='eps', handleA=A, handleB=B, val=ljeps)
ljcut.setParameter(param='sig', handleA=A, handleB=B, val=ljsig)


Activating the fix

#Activate fix
state.activateFix(ljcut)


## LAMMPS data file parameter order¶

rCut, eps, sig

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