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<span style="font-size: 10.5pt; font-family: "Times New Roman", Times, serif;" lang="en-US">Dear
</span><span style="font-size: 10.5pt; font-family: "Times New Roman", Times, serif;" lang="en-US">Kejiang,</span></div>
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You are welcome. For the first question, emas_cutoff is used to scale the mass of Fourier components of the wavefunction during the propagation of CPMD dynamics so as to compress the electronic power spectrum (i.e., the span of the lowest to the highest frequency
modes of the wavefunction); doing so allows one to choose a larger timestep during the propagation. Please see [Tassone, Mauri, and Car, Phys. Rev. B 50, 10561 (1994).] for details. When larger emass_cutoff is given (make sense upto ecutwfc), less mass preconditioning
is introduced so closer to the original CPMD result (though might require a smaller dt).</div>
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For the second, the core idea of CPMD is essentially to reproduce BOMD without the need to perform self-consistent field calculations beyond the first step (thus reducing the computational cost). For metallic systems, the validity of the BO approximation becomes
questionable with the closing bandgap. As such,<span style="font-size:12pt"> this topic is still a branch of active research and requires careful understanding of the simulation results even though both
</span>CPMD (may need electronic thermostats) and BOMD machinery can be practically performed.<span style="font-family: "Times New Roman", Times, serif; font-size: 12pt; color: rgb(0, 0, 0);"> You might want to consult a colleague familiar with ab initio simulations
of iron oxides and iron surfaces to make these decisions.<br>
</span></div>
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<span style="font-family: "Times New Roman", Times, serif; font-size: 12pt; color: rgb(0, 0, 0);"><br>
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<span style="font-family: "Times New Roman", Times, serif; font-size: 12pt; color: rgb(0, 0, 0);">Best,</span></div>
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<span style="font-family: "Times New Roman", Times, serif; font-size: 12pt; color: rgb(0, 0, 0);">Hsin-Yu</span></div>
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<div><span style="font-family:georgia,serif"><span style="color:rgb(102,102,102)">Hsin-Yu Ko</span></span></div>
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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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<b>Sent:</b> Friday, April 8, 2022 7:19 PM<br>
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<b>Subject:</b> [QE-users] 回复: 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 sharing the document.<br>
<br>
Now I understand the two questions much better, but I have two more questions to consult with you.<br>
<br>
The first is a small question:<br>
In the CPMD tutorial, it also suggests to tune ‘emass_cutoff’. What is the </span>
<span lang="EN-US" style="font-size:10.5pt; font-family:等线">physical philosophy to tune
</span><span lang="EN-US" style="font-size:10.5pt; font-family:等线">‘emass_cutoff’ ? Will a higher emass_cutoff produce a result more accurately?
<br>
<br>
The second is might a bigger question:<br>
I noticed in many places that we should use BOMD if we need to simulate metal system or systems with a band gap. What is the
</span><span lang="EN-US" style="font-size:10.5pt; font-family:等线">physical philosophy behind this practice? In fact, I am simulating the reduction of iron oxides by H2 to produce iron. There are band gaps for various kinds of iron oxides and metal (iron) will
be produced after reaction. Do I need to used BOMD instead of CPMD?<br>
<br>
Sorry that my academic background is not related with quantum physics or chemistry, I am not good at understand the complex equations related with quantum chemistry.<br>
<br>
Best regards,<br>
Kejiang<br>
<br>
</span><span lang="EN-US" style="font-size:10.5pt; font-family:等线">-------------------------------------------------<br>
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<br>
<a href="https://kejiangli.com/" target="_blank">https://kejiangli.com/</a></span></p>
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</span><b><span style="font-size:11.0pt; font-family:等线">代表 </span></b><span lang="EN-US" style="font-size:11.0pt; font-family:等线">Hsin-Yu Ko<br>
</span><b><span style="font-size:11.0pt; font-family:等线">发送时间<span lang="EN-US">:</span></span></b><span lang="EN-US" style="font-size:11.0pt; font-family:等线"> 2022</span><span style="font-size:11.0pt; font-family:等线">年<span lang="EN-US">4</span>月<span lang="EN-US">9</span>日<span lang="EN-US">
1:10<br>
</span><b>收件人<span lang="EN-US">:</span></b><span lang="EN-US"> 'Quantum ESPRESSO users Forum' <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] </span>
回复<span lang="EN-US">: 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"> </span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-family:"Times New Roman",serif; color:black">Dear
</span><span lang="EN-US" style="font-size:10.5pt; font-family:等线; color:black">Kejiang</span><span lang="EN-US" style="font-family:"Times New Roman",serif; color:black">,</span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-family:"Times New Roman",serif; color:black">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>. </span>
</p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-family:"Times New Roman",serif; color:black">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.</span></p>
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<p class="x_MsoNormal"><span lang="EN-US"> </span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-family:"Times New Roman",serif; color:black">Hope that helps,</span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="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-family:"Georgia",serif; color:#666666">Hsin-Yu Ko</span><span lang="EN-US" style="font-family:"Calibri",sans-serif; color:#888888"></span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-family:"Georgia",serif; color:#666666">Postdoctoral Research Fellow</span><span lang="EN-US" style="font-family:"Calibri",sans-serif; color:#888888"></span></p>
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<p class="x_MsoNormal"><span lang="EN-US" style="font-family:"Georgia",serif; color:#666666">Department of Chemistry and Chemical Biology<br>
Cornell University</span><span lang="EN-US" style="font-family:"Calibri",sans-serif; color:#888888"></span></p>
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<p class="x_MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt; font-family:"Calibri",sans-serif; color:black">From:</span></b><span lang="EN-US" style="font-size:11.0pt; font-family:"Calibri",sans-serif; 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 12:04 PM<br>
<b>To:</b> 'Quantum ESPRESSO users Forum' <<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] </span><span style="font-size:11.0pt; color:black">回复</span><span lang="EN-US" style="font-size:11.0pt; font-family:"Calibri",sans-serif; color:black">: Is it necessary to do "vc-relax" with pw.x before doing CPMD with cp.x ?</span><span lang="EN-US">
</span></p>
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<p class="x_MsoNormal"><span lang="EN-US"> </span></p>
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<p class="x_xmsonormal"><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 </span><span style="font-size:10.5pt; font-family:等线">‘<span lang="EN-US">emass</span>’<span lang="EN-US">, I have one more question:<br>
Is it a common practice or acceptable to change the value of </span>‘<span lang="EN-US">emass</span>’<span lang="EN-US"> 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></span><span lang="EN-US"></span></p>
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<p class="x_xmsonormal"><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">
<a href="mailto:users-bounces@lists.quantum-espresso.org">users-bounces@lists.quantum-espresso.org</a> <<a href="mailto:users-bounces@lists.quantum-espresso.org">users-bounces@lists.quantum-espresso.org</a>>
</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"> <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><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><span lang="EN-US"></span></p>
</div>
</div>
<p class="x_xmsonormal"><span lang="EN-US"> </span></p>
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<p class="x_xmsonormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Times New Roman",serif; color:black">Hi Kejiang,</span><span lang="EN-US"></span></p>
</div>
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<p class="x_xmsonormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Times New Roman",serif; color:black"> </span><span lang="EN-US"></span></p>
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<p class="x_xmsonormal"><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><span lang="EN-US"></span></p>
</div>
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<p class="x_xmsonormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Times New Roman",serif; color:black"> </span><span lang="EN-US"></span></p>
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<p class="x_xmsonormal"><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><span lang="EN-US"></span></p>
</div>
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<p class="x_xmsonormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Times New Roman",serif; color:black"> </span><span lang="EN-US"></span></p>
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<p class="x_xmsonormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Times New Roman",serif; color:black">Best,</span><span lang="EN-US"></span></p>
</div>
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<p class="x_xmsonormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Times New Roman",serif; color:black">Hsin-Yu</span><span lang="EN-US"></span></p>
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<p class="x_xmsonormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Georgia",serif; color:#666666">Hsin-Yu Ko</span><span lang="EN-US"></span></p>
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<p class="x_xmsonormal"><span lang="EN-US" style="font-size:12.0pt; font-family:"Georgia",serif; color:#666666">Postdoctoral Research Fellow</span><span lang="EN-US"></span></p>
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Cornell University</span><span lang="EN-US"></span></p>
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<p class="x_xmsonormal"><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">
</span></p>
<div>
<p class="x_xmsonormal"><span lang="EN-US"> </span></p>
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<div>
<p class="x_xxmsonormal"><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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