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: [email protected] <[email protected]> 代表 Hsin-Yu Ko 发送时间: 2022年4月8日 21:29 收件人: [email protected]; [email protected] 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 _____ From: users <[email protected] <mailto:[email protected]> > on behalf of [email protected] <mailto:[email protected]> <[email protected] <mailto:[email protected]> > Sent: Friday, April 8, 2022 2:44 AM To: [email protected] <mailto:[email protected]> <[email protected] <mailto:[email protected]> >; [email protected] <mailto:[email protected]> <[email protected] <mailto:[email protected]> > 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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