<div dir="auto">I recently run variable-cell optimization of various molecular crystals and I found xTB@CP2K ultra sensitive to EPS_DEFAULT. Tested from 1e-5 to 1e-24 (with EPS_SCF 1e-8), and no convergence happened. I just ended up with EPS_DEFAULT 1e-10 as a "gut" choice. Also, the behavior of xTB@CP2K is doutfull with MD even at ambiant conditions, where the converged volume is barely larget than at 0K. Depending on EPS_DEFAULT, it can even be smaller at ambient T. Weird. The behavior of DFTB2@CP2K is far better.<div dir="auto"><br></div><div dir="auto">I found that DFTB+ has other issues. xTB@DFTB+ has no convergence issue, but the recommended variable-cell optimization algorithm has flaws. The unit cell and a supercell does NOT always end up with related lattice parameters. The main issue is that some 90° angles are not preserved with DFTB+ whereas CP2K does (with no symmetry enforced, obviously). Some inconsistencies appears in DFTB+ with a lattice dimensions < 10 angstroms in the unit cell versus > 10 angstroms in the supercell. A proper tight mesh of k-points does not improve. So I'm afraid that xTB@DFTB+ (or DFTB+, actually) cannot be a relevant choice for crystal structure predictions, for instance.</div><div dir="auto"><br></div><div dir="auto">xTB may be unreliable with CP2K and DFTB+, but for the different reasons above. You can check these weird behaviors with your own crystals of interest.</div><div dir="auto"><br></div><div dir="auto">Xavier</div><div dir="auto"><br></div><div dir="auto"><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">Le lun. 5 sept. 2022, 3:59 AM, Jürg Hutter <<a href="mailto:hutter@chem.uzh.ch">hutter@chem.uzh.ch</a>> a écrit :<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi<br>
<br>
thank you for testing. Could you send a break down of the energies for the LiF molecule for<br>
the two codes? That might help to recognize the source of the difference.<br>
<br>
regards<br>
<br>
JH<br>
<br>
________________________________________<br>
From: <a href="mailto:cp2k@googlegroups.com" target="_blank" rel="noreferrer">cp2k@googlegroups.com</a> <<a href="mailto:cp2k@googlegroups.com" target="_blank" rel="noreferrer">cp2k@googlegroups.com</a>> on behalf of Magnus Rahm <<a href="mailto:magnus@compulartech.com" target="_blank" rel="noreferrer">magnus@compulartech.com</a>><br>
Sent: Monday, September 5, 2022 8:40 AM<br>
To: cp2k<br>
Subject: [CP2K:17599] Re: Large discrepancy in xTB results from CP2K vs DFTB+<br>
<br>
For the record, the problem is the same in CP2K version 2022.1.<br>
<br>
On Thursday, September 1, 2022 at 12:48:35 PM UTC+2 Magnus Rahm wrote:<br>
Dear all,<br>
<br>
I want to use CP2K (version 8.2, trying to get a more recent version compiled) together with xTB for a crystal containing Li and O. I get strange results already for a simple LiO2 crystal:<br>
<br>
* There is a very large discrepancy compared to DFTB+ (version 22.1).<br>
* Mulliken charges tend to be large, meaning that CHECK_ATOMIC_CHARGES stops the SCF. If I turn it off, the system tends to converge systematically to values just outside the "chemical range". Mulliken charges obtained by DFTB+ are significantly smaller (and within "chemical range").<br>
* The energy-volume curve looks strange and very different from DFTB+.<br>
<br>
I have tried converging with respect to system size and the EWALD / ALPHA and GMAX parameters, but they have only a marginal impact. I have tried similar calculations for a number of periodic systems. Sometimes I get agreement, sometimes not. I also tried calculations for CO and NO molecules which agree perfectly between CP2K and DFTB+, whereas an artificial LiF molecule does not.<br>
<br>
A perhaps related issue was reported in <a href="https://groups.google.com/g/cp2k/c/oFwgGcQuySs" rel="noreferrer noreferrer" target="_blank">https://groups.google.com/g/cp2k/c/oFwgGcQuySs</a> but the solutions suggested there did not solve my problem.<br>
<br>
I attach input scripts for CP2K and DFTB+, as well as a figure showing the E-V curve for LiO2 obtained with CP2K and DFTB+. I'm new to CP2K, DFTB+ and xTB so I suspect I have made some simple mistake, and any advice is appreciated.<br>
<br>
Kind regards,<br>
Magnus Rahm<br>
<br>
<br>
<br>
<br>
<br>
<br>
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