[QE-users] What to modify in "dynamics_module.f90" for making constant zero pressure during NVT simulation(pw.x)

[Stefano de Gironcoli]

Dear Jayraj,

adding to what Lorenzo already replied, P and V as well as E and T are 
conjugated quantities: if you fix one the other fluctuates (of an amount 
\propto 1/sqrt N that vanishes in the thermodynamic limit).

so you can have NVE simulations where the average kinetic energy is 
related to the system temperature but fluctuates in time, or using a 
thermostat you can fix the system temperature (sampling NVT ensemble) 
but you give up energy conservation.

the same for V and P, you can have NVT and NPT, or NVE and NPH (because 
the conserved quantity is in this case the enthalpy). The larger the 
system the smaller the fluctuations but for finite number of particles 
the fluctuations are always present (and the thermostats and barostats 
do have their own degrees of freedom whose contribution become 
negligible in the thermodynamic limit).

stefano

On 16/03/24 13:56, Lorenzo Paulatto wrote:
> On 16/03/2024 12:17, Jayraj Anadani wrote:
>> hello Paulatto,
>> but in cell_dynamics all options for pressure control are for vc-md 
>> and vc-relax. and i want to control pressure during 
>> ion_temperature=reduce-T option.
>
> Dear Jayraj,
>
> it is not very clear what you want to do. You can have constant NVT, 
> or NVE or NPT, or NPE. But it looks to me that you want constant 
> instantaneous P and V, which is impossible as P is a macroscopic 
> quantity, it only makes sense on average.
>
> kind regards
>
>
>>
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> -- 
> Dr. Lorenzo Paulatto
> IdR @ IMPMC - CNRS UMR 7590 & Sorbonne Université
> phone: +33 (0)1 442 79822 / skype: paulatz
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>
> _______________________________________________
> The Quantum ESPRESSO community stands by the Ukrainian
> people and expresses its concerns about the devastating
> effects that the Russian military offensive has on their
> country and on the free and peaceful scientific, cultural,
> and economic cooperation amongst peoples
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing listusers at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users
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