From owner-chemistry@ccl.net Fri Sep 6 02:00:01 2019 From: "Susi Lehtola susi.lehtola*_*helsinki.fi" To: CCL Subject: CCL:G: Structure breaks even after started with correct bond length Message-Id: <-53838-190906015728-1240-rJ6uWjhxiWd+5G17dcRIog]=[server.ccl.net> X-Original-From: Susi Lehtola Content-Language: en-US Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=utf-8; format=flowed Date: Fri, 6 Sep 2019 08:57:08 +0300 MIME-Version: 1.0 Sent to CCL by: Susi Lehtola [susi.lehtola~!~helsinki.fi] On 9/5/19 11:30 PM, Robert Molt r.molt.chemical.physics*|*gmail.com wrote: > Dr. Lehtola: > > Given that the user's question is what has gone wrong with the > /geometry/, effectively, isn't it somewhat inappropriate to say that > post HF methods require basis sets of TZ or larger? I believe that > post-HF methods get perfectly respectable geometries using DZ basis > sets; I've never encountered a time that the optimized geometry was not > reasonable for a single-reference molecule. I certainly agree that the > energy may require TZ or more, but if the molecule breaks up, it speaks > to a larger problem with the user's starting geometry or "single zeta" > basis. Dr Molt, that depends on the basis set. Minimal basis sets exxagerate correlation effects, since the one-electron part is described at a poor level. Since the system was not given, I don't know whether it is single reference or not, as I wrote in the answer. Problems in the correlation treatment were apparent due to the Gaussian error of too large mixing with the frozen core. My suggestion was to switch to DFT which is usually more reliable in small basis sets. If the problem is still reproducible, then there might be something else wrong in the calculation. My suggestion still is to start by reproducing the CP2k structure. If that works, then lose the periodic boundary conditions and see what happens. If that works, then do post-HF methods. -- ------------------------------------------------------------------ Mr. Susi Lehtola, PhD Junior Fellow, Adjunct Professor susi.lehtola!A!alumni.helsinki.fi University of Helsinki http://susilehtola.github.io/ Finland ------------------------------------------------------------------ Susi Lehtola, dosentti, FT tutkijatohtori susi.lehtola!A!alumni.helsinki.fi Helsingin yliopisto http://susilehtola.github.io/ ------------------------------------------------------------------