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Re: [lammps-users] NPT pressure problem
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Re: [lammps-users] NPT pressure problem

From: Giacomo Fiorin <giacomo.fiorin@...24...>
Date: Thu, 22 Mar 2018 10:40:09 -0400

Hello Anderson, high fluctuations of the virial stress are common.  This is discussed in the literature, textbooks, and you can verify that in example systems that use fix NPT.

For more useful feedback, you need to ask a direct question on a specific problem that has not been discussed already in the literature or LAMMPS documentation.


On Thu, Mar 22, 2018 at 1:06 AM, Jeams Anderson <anddersonnyc@...24...> wrote:
Dear All

I am trying to calculate 5*5*5 nm box of electrolyte solution. The solutes are mixed with solven by considering 1M concentration. However, to gain bulk properties I need to get equilibrium  system  at 1 atm pressure. Therefore, First I run the NVE with langevine fix. Then I use press/berendsen and temp/berendsen to get the 1 atm pressure. Then I run the npt. But I really got confuse with some issues 

1. When I run press/berendsen and temp/berendsen after NVE I got the my desired temp and pressure with a few steps. After that all values got constant ( e.g Pot_Eng, Tot_Eng everything). I am wondering is this OK for press/berendsen and temp/berendsen? ( Since I could not find any example for this fix I got confused) 

2. After press/berendsen and temp/berendsen calcualtion when I run npt with pressure 1 atm, my pressure changed with a large amount ( e.g from 1.02 to -895.23 ) and then pressure fluctuate +- 600 at every 100 steps.

***  Units is real** and command is like this

fix 1        nve all 
fix  2       all langevin 303 303 100 67891

Then for berendsen  

fix              1 all press/berendsen iso 1.0 1.0 1000 modulus 1000 
fix              2  all temp/berendsen 303 303 100      

Then for npt calculation 

fix              1  all npt temp 303 303 100 iso 1.0 1.0 1000

I reach at this stage of calculation by using manual and mail lists. And I also tried to find a reasonable solution for present stage. However, I failed to make it, therefore, it will be very helpful if experts show me the way thus I could solve the problem or sort out my mistakes.

Thank in advance 



Houston University

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Giacomo Fiorin
Associate Professor of Research, Temple University, Philadelphia, PA
Contractor, National Institutes of Health, Bethesda, MD