[lammps-users] which ensemble should I use (Manoj Warrier)

Dear Manoj,
I am a beginner in this field,but I want to mention few things.Repeated mails encouraged me to reply on this issue.You have written-

“Carrying out an MD simulation usually consists of two main parts:
1) You set up the system … i.e. assign particle positions, velocities, etc and relax the system.
2) Choose an appropriate ensemble, and sample the required physical quantities of interest.
An NPT is usually required in part (1) above. If you are simulating any system with a surface (say surface bombardment at room temperature and pressure), you would want to do an NPT of the system so that the positions chosen by you are consistent with the interatomic potential and would expand / contract such that the pressure of the system oscillates around the ambient pressure. Because you have a surface at one plane using PBC is ruled out.”

Part (2) depends on what one wants to measure and is not relevant to our discussion.

Regarding 1st point,that is relaxation to allow the system to reach equilibrium(minimization).There is no particular relation with NPT. It depends on you and the appropriate procedure you want to use for this purpose.You can do it anyway,but if you don’t follow accordingly,obtained data will be incorrect.If you don’t use that,your contrasted model will have some added kinetic energy which is not desireable for the MD simulation.Because it will be added to the fixes you impose next. Consider your system as a classical thermodynamical system.then relax it accordingly.

Regarding 2nd point,statistical ensembles will depend on the system you are trying to simulate. Proper ensemble will give you precise output. You may also use fixes invoked in lammps with your desired parameters to get appropriate output.Say, a indentation test,can be done using only fix nve depending on the criteria.May be you need to control the temperature like coupling your model to a heat sink.you can do that by fixes.or put some constraints.It depends totally on your motivation.There is nothing wrong or there is nothing right.You have to balance the commands and put it accordingly to get desired simualtion.


Hope it answers the questions.If I am wrong,please make me correct.

Regards,
Hanif.
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