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


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Thu, 10 Aug 2017 11:17:32 -0400



On Thu, Aug 10, 2017 at 6:57 AM, <sindhana@...7053...> 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
radius used for potential.

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


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Dr. Axel Kohlmeyer  akohlmey@...92......  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.