[QE-users] Is it necessary to do "vc-relax" with pw.x before doing CPMD with cp.x ?

[Hsin-Yu Ko]

Hi Kejiang,

Structural relaxation in cp.x (step 2 as you mentioned) can involve the cell degrees of freedom (in addition to the ionic ones) and thus is in principle equivalent to doing "vc-relax" in pw.x. As such, it is unnecessary to do a "vc-relax" with pw.x.

In practice, proper parameter setting relies on some intuition of the targeted physical system. I think the difficulty in force convergence probably is related to "emass" and "dt" (e.g., when the emass is set too large, ionic motion during relaxation could fictitiously heat up the electronic degrees of freedom and lead to fluctuations in forces). Nevertheless, detailed input/output information would be needed for more definitive comments.

Best,
Hsin-Yu

--
Hsin-Yu Ko
Postdoctoral Research Fellow
Department of Chemistry and Chemical Biology
Cornell University

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Sent: Friday, April 8, 2022 2:44 AM
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Subject: [QE-users] Is it necessary to do "vc-relax" with pw.x before doing CPMD with cp.x ?


Dear QE-Users,

I am doing CPMD with cp.x for iron oxide system following the official tutorial which introduced two steps (Step1: Reaching the electronic ground state; Step 2: Relax the system) before doing cpmd.

However, in both the electronic minimization and ionic minimization processes, I found that the force cannot converge (forc_conv_thr=1.0D-3 cannot be satisfied), while it is relative easy for electronic energy and total energy to converge (ekin_conv_thr=1.0D-6, etot_conv_thr= 1.0D-4).

I am wondering whether this is caused by that the initial crystal structure is not fully relaxed with the current pseudopotential. Is it necessary to do "vc-relax" with pw.x before doing CPMD with cp.x ?  If yes, which parameters should be kept to be the same in both pw.x and cp.x?

Any comment or suggestion will be much appreciated.

Thanks,
Kejiang

-------------------------------------------------
Dr. Kejiang Li
School of Metallurgical and Ecological Engineering,
University of Science and Technology Beijing
Xueyuan Rd., Haidian District, Beijing 100083, P. R. China
https://kejiangli.com/


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