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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: sindhana@...7053...
Date: Fri, 11 Aug 2017 11:32:05 +0530

Dear Prof. Axel Kohlmeyer,

The pressure correction used is for the plain cutoff and does not take
into account the smooth truncation of the potential.

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

> 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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>>
>>
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>>
>
>
>
> --
> Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
> College of Science & Technology, Temple University, Philadelphia PA, USA
> International Centre for Theoretical Physics, Trieste. Italy.
>