LAMMPS WWW Site - LAMMPS Documentation - LAMMPS Mailing List Archives
Re: [lammps-users] Lennard-Jones 2D pressure changes with potential cut off

Re: [lammps-users] Lennard-Jones 2D pressure changes with potential cut off

 From: Axel Kohlmeyer Date: Thu, 10 Aug 2017 11:17:32 -0400

On Thu, Aug 10, 2017 at 6:57 AM, wrote:
Dear All,

Version of LAMMPS: 5 May 2012

I ran a 2D simulation of Lennard-Jones particles at various temperatures
and densities using an NVT ensemble and noted that the pressure (after
including long-range correction) increases with increasing cut off length
used for the potential.

pair potential used: lj/charmm/coul/charmm
Charge on particles: 0

The system contains 2500 particles and was run with periodic boundary
conditions. All data stated below are in Lennard-Jones units. I used the
_expression_ given below for long-range correction.

P_correction=12/5(pi)*(rho)^2*[(1/r_i)^10-5/4*(1/r_i)^4]

​is this correction specifically for the CHARMM variant of the lennard-jones interaction which uses a smooth truncation of the potential?
or for plain cutoff?
please also note, that if you want to *exactly* reproduce CHARMM, you need to use a slightly different switching function, i.e. the one implemented in the lj/charmmfsw/coul/*
the one in lj/charmm/coul/* corresponds to an extremely old variant of CHARMM.

axel.

rho=density, T=temperature, P=pressure, delta P=uncertainty in pressure,
pi=3.14, r_i=inner cut-off radius used for potential, r_o=outer cut-off

T       rho     r_i     r_o     P       delta P
0.5     0.75    3.0     3.1     0.2936  0.0014
3.5     3.6     0.3065  0.0016
4.0     4.1     0.3216  0.0007
4.5     4.6     0.3230  0.0024

0.7     0.75    3.0     3.1     1.1724  0.0026
3.5     3.6     1.1874  0.0028
4.0     4.1     1.2020  0.0031
4.5     4.6     1.2037  0.0023

1.0     0.75    3.0     3.1     2.3817  0.0037
3.5     3.6     2.3951  0.0019
4.0     4.1     2.4063  0.0037
4.5     4.6     2.4116  0.0030

1.0     0.50    3.0     3.1     0.5510  0.0010
3.5     3.6     0.5589  0.0008
4.0     4.1     0.5648  0.0008
4.5     4.6     0.5658  0.0004

1.4     0.10    3.0     3.1     0.1386  0.0001
3.5     3.6     0.1390  0.0001
4.0     4.1     0.1391  0.0001
4.5     4.6     0.1392  0.0001

Regards,
Sindhana Selvi P.S.
Masters student (Physics)
Indian Institute of Science

--
This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most