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

[likejiang at ustb.edu.cn]

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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