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Dear <span style="font-size:10.5pt;font-family:等线" lang="en-US">Kejiang</span>,</div>
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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 <a href="https://juser.fz-juelich.de/record/44687/files/NIC-Band-1.pdf#page=311">
https://juser.fz-juelich.de/record/44687/files/NIC-Band-1.pdf#page=311</a>. <br>
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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.<br>
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Hope that helps,</div>
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Hsin-Yu<br>
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<div><span style="font-family:georgia,serif"><span style="color:rgb(102,102,102)">Hsin-Yu Ko</span></span></div>
<div><span style="font-family:georgia,serif"><span style="color:rgb(102,102,102)">Postdoctoral Research Fellow<br>
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<span style="font-family:georgia,serif"><span style="color:rgb(102,102,102)">Department of Chemistry and Chemical Biology<br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> users <users-bounces@lists.quantum-espresso.org> on behalf of likejiang@ustb.edu.cn <likejiang@ustb.edu.cn><br>
<b>Sent:</b> Friday, April 8, 2022 12:04 PM<br>
<b>To:</b> 'Quantum ESPRESSO users Forum' <users@lists.quantum-espresso.org>; users-request@lists.quantum-espresso.org <users-request@lists.quantum-espresso.org><br>
<b>Subject:</b> [QE-users] 回复: Is it necessary to do "vc-relax" with pw.x before doing CPMD with cp.x ?</font>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-size:10.5pt; font-family:等线">Dear Hsin-Yu,<br>
<br>
Thanks a lot for your explanation and suggestion.<br>
<br>
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.<br>
<br>
Regarding ‘emass’, I have one more question:<br>
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? <br>
<br>
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?<br>
<br>
Thanks again for your kind help.<br>
<br>
Best regards,<br>
Kejiang</span></p>
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<p class="x_MsoNormal"><b><span style="font-family:等线">发件人<span lang="EN-US">:</span></span></b><span lang="EN-US" style="font-family:等线"> users-bounces@lists.quantum-espresso.org <users-bounces@lists.quantum-espresso.org>
</span><b><span style="font-family:等线">代表 </span></b><span lang="EN-US" style="font-family:等线">Hsin-Yu Ko<br>
</span><b><span style="font-family:等线">发送时间<span lang="EN-US">:</span></span></b><span lang="EN-US" style="font-family:等线"> 2022</span><span style="font-family:等线">年<span lang="EN-US">4</span>月<span lang="EN-US">8</span>日<span lang="EN-US"> 21:29<br>
</span><b>收件人<span lang="EN-US">:</span></b><span lang="EN-US"> users@lists.quantum-espresso.org; users-request@lists.quantum-espresso.org<br>
</span><b>主题<span lang="EN-US">:</span></b><span lang="EN-US"> Re: [QE-users] Is it necessary to do "vc-relax" with pw.x before doing CPMD with cp.x ?</span></span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Times New Roman",serif; color:black">Hi Kejiang,</span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Times New Roman",serif; color:black">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.</span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Times New Roman",serif; color:black">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.</span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Times New Roman",serif; color:black">Best,</span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Times New Roman",serif; color:black">Hsin-Yu</span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Georgia",serif; color:#666666">Hsin-Yu Ko</span><span lang="EN-US" style="font-size:12.0pt; color:#888888"></span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Georgia",serif; color:#666666">Postdoctoral Research Fellow</span><span lang="EN-US" style="font-size:12.0pt; color:#888888"></span></p>
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Cornell University</span><span lang="EN-US" style="font-size:12.0pt; color:#888888"></span></p>
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<p class="x_MsoNormal"><b><span lang="EN-US" style="color:black">From:</span></b><span lang="EN-US" style="color:black"> users <<a href="mailto:users-bounces@lists.quantum-espresso.org">users-bounces@lists.quantum-espresso.org</a>> on behalf of
<a href="mailto:likejiang@ustb.edu.cn">likejiang@ustb.edu.cn</a> <<a href="mailto:likejiang@ustb.edu.cn">likejiang@ustb.edu.cn</a>><br>
<b>Sent:</b> Friday, April 8, 2022 2:44 AM<br>
<b>To:</b> <a href="mailto:users@lists.quantum-espresso.org">users@lists.quantum-espresso.org</a> <<a href="mailto:users@lists.quantum-espresso.org">users@lists.quantum-espresso.org</a>>;
<a href="mailto:users-request@lists.quantum-espresso.org">users-request@lists.quantum-espresso.org</a> <<a href="mailto:users-request@lists.quantum-espresso.org">users-request@lists.quantum-espresso.org</a>><br>
<b>Subject:</b> [QE-users] Is it necessary to do "vc-relax" with pw.x before doing CPMD with cp.x ?</span><span lang="EN-US">
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<p class="x_xmsonormal"><span lang="EN-US">Dear QE-Users,<br>
<br>
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.<br>
<br>
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). <br>
<br>
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?<br>
<br>
Any comment or suggestion will be much appreciated.<br>
<br>
Thanks,<br>
Kejiang</span><b><span lang="EN-US" style="font-size:11.0pt"><br>
<br>
</span></b><span lang="EN-US">-------------------------------------------------<br>
</span><span lang="EN-US" style="font-size:9.0pt">Dr. Kejiang Li<br>
School of Metallurgical and Ecological Engineering,<br>
University of Science and Technology Beijing<br>
Xueyuan Rd., Haidian District, Beijing 100083, P. R. China</span><span lang="EN-US" style="font-size:7.5pt"><br>
<a href="https://kejiangli.com/" target="_blank">https://kejiangli.com/</a></span><span lang="EN-US"></span></p>
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