[QE-users] Best way to choose dt and nstep in AIMD/CPMD simulation using cp.x code in quantum espresso

[Jayraj Anadani]

In my case, I have a system of 54 zrcu atoms. i want to melt it at higher
temperatures above 2000K and then i will do quenching to get amorphorse
metallic alloys state.
But shorter time step in cg method give more time consuming. So i choose dt
= between 30 to 60 and in order to get equilibrium at high temperatures i
took nstep = above 500.
For equilibrium i have to run continue till 3 to 5 ps to get randomisation
so it took large time to complete.
Any suggestions ?

Thank you
Jayraj Anadani


On Thu, 15 Sep, 2022, 1:28 pm Kazume NISHIDATE, <nisidate at iwate-u.ac.jp>
wrote:

> Dear, Jayraj Anadani
>
> > According to the user guide of cp.x we can use a larger time step
> > "dt" for electron dynamics = "cg" but for my system of 54 atoms
> > (metallic) at 2000 K temperature, it takes a lot of time to take 500
> > steps for 36 core, 72 nodes.
>
> The temperature 2000 K is very high and the ions must have high
> velocities. It means that the ionic positions at the next time step
> will largely different from the current ones. The convergence of the
> wave functions will need many 'cg' steps accordingly.
>
> So, you should set short 'dt' to ensure the first convergence of the
> wave functions.
>
> > 1) is there any way to balancing between timestep "dt" and number of
> >  CP-MD steps "nstep" to achieve fast simulation ?,
>
> It is worth to try the shorter time intervals (dt) with fixing other
> parameters such as the total time steps.
>
> > 2) In comparison with VASP most of the simulation like NVT, NPT, NVE
> >  took 3 to 5 ps to equilibrate at a certain temperature so is there
> >  any strategy to speed up the calculation ?
>
> Generally, 3 to 5 ps is enough to achieve equilibrium state.  But I do
> not understand your system and not sure if it is appropriate to your
> case.
>
> > 2022/09/15 16:18、Jayraj Anadani <jayrajanadani at spuvvn.edu>のメール:
> >
> > Hello QE Community,
> > According to the user guide of cp.x we can use a larger time step "dt"
> for electron dynamics = "cg" but for my system of 54 atoms (metallic) at
> 2000 K temperature, it takes a lot of time to take 500 steps for 36 core,
> 72 nodes.
>
> 西館数芽
> Kazume NISHIDATE Ph.D
>
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