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

[Hsin-Yu Ko]

Dear Kejiang,

Good questions! The physical idea of emass is to slow down the timescale of the fictitious electronic motions (to allow larger integration timestep, dt, for propagating CPMD). However, emass has to be chosen to maintain good adiabatic separation between fictitious electronic and ionic degrees of freedom such that the electronic wavefunctions remain close to the ground state (BO surface) during the CPMD propagation. A very nice explanation (including how parameters should be chosen) by Marx and Hutter can be found in Secs. 2.4.3-2.4.4 at https://juser.fz-juelich.de/record/44687/files/NIC-Band-1.pdf#page=311.

For your second question, it depends on what properties one wants to sample via CPMD. For instance, the structural properties sampled in the canonical (NVT) ensemble is independent of ionic mass (assuming that the nuclei are classical particles) since the resulting/equilibrated trajectory follows the Boltzman distribution (independent of ionic mass). However, arbitrary choices of ionic mass certainly defeat the purpose if the goal is to extract dynamical properties.

Hope that helps,
Hsin-Yu

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

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From: users <users-bounces at lists.quantum-espresso.org> on behalf of likejiang at ustb.edu.cn <likejiang at ustb.edu.cn>
Sent: Friday, April 8, 2022 12:04 PM
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Subject: [QE-users] 回复: Is it necessary to do "vc-relax" with pw.x before doing CPMD with cp.x ?


Dear Hsin-Yu,

Thanks a lot for your explanation and suggestion.

As per your comments, it might be due to high emass since I used the default emass (400). I will try to decrease emass (300, 200, 100) to see whether the force can converge.

Regarding ‘emass’, I have one more question:
Is it a common practice or acceptable to change the value of ‘emass’ in different stages of the simulation, i.e., emass (50) in the electronic minimization stage, emass (200) in the ionic minimization stage and emass (400) in the CPMD stage.  If yes, what is the physical philosophy behind or support this practice?

I addition, the official guide also suggests to decrease the ionic mass the speed up the computation. Why is this physically acceptable since a specific kind of element should has a specific mass?

Thanks again for your kind help.

Best regards,
Kejiang



发件人: users-bounces at lists.quantum-espresso.org <users-bounces at lists.quantum-espresso.org> 代表 Hsin-Yu Ko
发送时间: 2022年4月8日 21:29
收件人: users at lists.quantum-espresso.org; users-request at lists.quantum-espresso.org
主题: Re: [QE-users] Is it necessary to do "vc-relax" with pw.x before doing CPMD with cp.x ?



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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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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