<div dir="ltr">Dear QE developers and users,<br><br>Recently, I am trying to run MD calculations (NVT ensemble) for a solid at high pressure high temperature using Andersen thermostat. The setting of my calculations is thus calculation= 'md', ion_temperature= 'andersen', and a temperature suitable for my system. Given the wall time limit, I can only run 2000 steps for each job (~2000 fs). To reach equilibrium, I have to run at least six jobs (> 1.2 ps). In these calculations, I noticed that both the stress tensor and temperature exhibit an "artificial" periodicity of 2000 time steps, same as the setting of "nstep" in my calculations. I thus suspect that Andersen collisions in my calculations (and thus QE) may be non-random. <div><br></div><div>To further investigate (and better showcase) this issue in QE, I performed a series of tests on a simpler system: 8-atom fcc Si (similar to example03). For the thermostat, the setting is ion_temperature= 'andersen', tempw= 300, and nraise= 10. In these test calculations, I find the same problem persists, namely, an artificial periodicity same as "nstep" (500 in these test calculations), as further described below. (Note: A few example input/output files are attached. All these test calculations are performed using QE 7.2. They can all be completed within one minute using one single CPU, if anyone is interested in trying).</div><div><br></div><div>In the first round (from scratch, pwscf.md.1.*), everything seems normal. The problem occurs in the "restart" rounds (pwscf.md.2.*, and 3, 4, ...). One can easily check the occurence of Andersen collisions in each round by "grep -e Entering -e collision" pwscf.md.?.out. By doing so, one can easily notice that the occurrence of Andersen collisions in the restart rounds are no longer random. For example, in round 2 (pwscf.md.2.out, timestep 501–1000), we have</div><div><br></div><div>     Entering Dynamics:    iteration =   501<br>     Andersen thermostat:    2 collisions<br>     Entering Dynamics:    iteration =   502<br>     Entering Dynamics:    iteration =   503<br>     Entering Dynamics:    iteration =   504<br>     Entering Dynamics:    iteration =   505<br>     Entering Dynamics:    iteration =   506<br>     Andersen thermostat:    2 collisions<br>     Entering Dynamics:    iteration =   507<br>     Andersen thermostat:    1 collisions<br>     Entering Dynamics:    iteration =   508<br>     Andersen thermostat:    1 collisions<br>     Entering Dynamics:    iteration =   509<br>     Andersen thermostat:    1 collisions<br>     Entering Dynamics:    iteration =   510<br>     Entering Dynamics:    iteration =   511<br>     Andersen thermostat:    1 collisions<br></div><div>     .....</div><div><br></div><div>In round 3 (pwscf.md.3.out, timestep 1001–1501), Andersen collisions occur in the same respective time steps as in round 2:</div><div><br></div><div>     Entering Dynamics:    iteration =  1001<br>     Andersen thermostat:    2 collisions<br>     Entering Dynamics:    iteration =  1002<br>     Entering Dynamics:    iteration =  1003<br>     Entering Dynamics:    iteration =  1004<br>     Entering Dynamics:    iteration =  1005<br>     Entering Dynamics:    iteration =  1006<br>     Andersen thermostat:    2 collisions<br>     Entering Dynamics:    iteration =  1007<br>     Andersen thermostat:    1 collisions<br>     Entering Dynamics:    iteration =  1008<br>     Andersen thermostat:    1 collisions<br>     Entering Dynamics:    iteration =  1009<br>     Andersen thermostat:    1 collisions<br>     Entering Dynamics:    iteration =  1010<br>     Entering Dynamics:    iteration =  1011<br>     Andersen thermostat:    1 collisions<br>    </div><div>In round 4 (pwscf.md.4.out, timestep 1501–2000), the same pattern persists:</div><div><br></div><div>     Entering Dynamics:    iteration =  1501<br>     Andersen thermostat:    2 collisions<br>     Entering Dynamics:    iteration =  1502<br>     Entering Dynamics:    iteration =  1503<br>     Entering Dynamics:    iteration =  1504<br>     Entering Dynamics:    iteration =  1505<br>     Entering Dynamics:    iteration =  1506<br>     Andersen thermostat:    2 collisions<br>     Entering Dynamics:    iteration =  1507<br>     Andersen thermostat:    1 collisions<br>     Entering Dynamics:    iteration =  1508<br>     Andersen thermostat:    1 collisions<br>     Entering Dynamics:    iteration =  1509<br>     Andersen thermostat:    1 collisions<br>     Entering Dynamics:    iteration =  1510<br>     Entering Dynamics:    iteration =  1511<br>     Andersen thermostat:    1 collisions<br><br></div><div>As shown above, in all the restart runs (500 steps each), Andersen collisions occur in the same respective time steps with the same number of collisions. For example, two collisions occur at time steps 501, 1001, and 1501, no collision in steps 502–505, 1002–1005, 1502–1505, two collisions at 506, 1006, and 1506, ...etc. In fact, throughout all the restart rounds (rounds 2–10), the same problem occurs. In other words, Andersen collisions in round 2–10 (time steps 501–5000) occur in a repetitive manner with a period of 500 time steps. </div><div><br></div><div>I am wondering how this can be fixed? Or maybe I am wrong, as this "artificial periodicity" is not a problem at all?</div><div><br></div><div>I truly appreciate any comments and suggestions.</div><div><br></div><div>Best regards,</div><div><br></div><div>Han</div><div><br><div class="gmail_chip gmail_drive_chip" style="width:396px;height:18px;max-height:18px;background-color:#f5f5f5;padding:5px;color:#222;font-family:arial;font-style:normal;font-weight:bold;font-size:13px;border:1px solid #ddd;line-height:1"><a href="https://drive.google.com/file/d/1ialx9iPnx8n9Dd6VAIogt_mgy30qmEXq/view?usp=drive_web" target="_blank" style="display:inline-block;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;text-decoration:none;padding:1px 0px;border:none;width:100%" aria-label="pwscf.md.1.in"><img style="vertical-align: bottom; border: none;" src="https://drive-thirdparty.googleusercontent.com/16/type/application/octet-stream"> <span dir="ltr" style="color:#15c;text-decoration:none;vertical-align:bottom">pwscf.md.1.in</span></a></div><br><div class="gmail_chip gmail_drive_chip" style="width:396px;height:18px;max-height:18px;background-color:#f5f5f5;padding:5px;color:#222;font-family:arial;font-style:normal;font-weight:bold;font-size:13px;border:1px solid #ddd;line-height:1"><a href="https://drive.google.com/file/d/1GdPyB4X2_eYrAZl1WEoJZNQmEeDSmqqn/view?usp=drive_web" target="_blank" style="display:inline-block;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;text-decoration:none;padding:1px 0px;border:none;width:100%" aria-label="pwscf.md.1.out"><img style="vertical-align: bottom; border: none;" src="https://drive-thirdparty.googleusercontent.com/16/type/application/octet-stream"> <span dir="ltr" style="color:#15c;text-decoration:none;vertical-align:bottom">pwscf.md.1.out</span></a></div><br><div class="gmail_chip gmail_drive_chip" style="width:396px;height:18px;max-height:18px;background-color:#f5f5f5;padding:5px;color:#222;font-family:arial;font-style:normal;font-weight:bold;font-size:13px;border:1px solid #ddd;line-height:1"><a href="https://drive.google.com/file/d/15ZgVyb00FJeUtZKnDPKAVny9Y8hO4E0b/view?usp=drive_web" target="_blank" style="display:inline-block;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;text-decoration:none;padding:1px 0px;border:none;width:100%" aria-label="pwscf.md.2.in"><img style="vertical-align: bottom; border: none;" src="https://drive-thirdparty.googleusercontent.com/16/type/application/octet-stream"> <span dir="ltr" style="color:#15c;text-decoration:none;vertical-align:bottom">pwscf.md.2.in</span></a></div><br><div class="gmail_chip gmail_drive_chip" style="width:396px;height:18px;max-height:18px;background-color:#f5f5f5;padding:5px;color:#222;font-family:arial;font-style:normal;font-weight:bold;font-size:13px;border:1px solid #ddd;line-height:1"><a href="https://drive.google.com/file/d/1xo_rWYX3FTcgcouHEIAa6_xkpAhDgc7I/view?usp=drive_web" target="_blank" style="display:inline-block;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;text-decoration:none;padding:1px 0px;border:none;width:100%" aria-label="pwscf.md.2.out"><img style="vertical-align: bottom; border: none;" src="https://drive-thirdparty.googleusercontent.com/16/type/application/octet-stream"> <span dir="ltr" style="color:#15c;text-decoration:none;vertical-align:bottom">pwscf.md.2.out</span></a></div><br><div class="gmail_chip gmail_drive_chip" style="width:396px;height:18px;max-height:18px;background-color:#f5f5f5;padding:5px;color:#222;font-family:arial;font-style:normal;font-weight:bold;font-size:13px;border:1px solid #ddd;line-height:1"><a href="https://drive.google.com/file/d/1KwDpT8Exm5ClCmZL6zpMdao5-IpJHv-p/view?usp=drive_web" target="_blank" style="display:inline-block;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;text-decoration:none;padding:1px 0px;border:none;width:100%" aria-label="pwscf.md.3.in"><img style="vertical-align: bottom; border: none;" src="https://drive-thirdparty.googleusercontent.com/16/type/application/octet-stream"> <span dir="ltr" style="color:#15c;text-decoration:none;vertical-align:bottom">pwscf.md.3.in</span></a></div><br><div class="gmail_chip gmail_drive_chip" style="width:396px;height:18px;max-height:18px;background-color:#f5f5f5;padding:5px;color:#222;font-family:arial;font-style:normal;font-weight:bold;font-size:13px;border:1px solid #ddd;line-height:1"><a href="https://drive.google.com/file/d/1R-mdwRht4XdJ-sxym9Q5TkjPf5ZesELT/view?usp=drive_web" target="_blank" style="display:inline-block;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;text-decoration:none;padding:1px 0px;border:none;width:100%" aria-label="pwscf.md.3.out"><img style="vertical-align: bottom; border: none;" src="https://drive-thirdparty.googleusercontent.com/16/type/application/octet-stre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