Dear Aleksandra > I tried both ways and came to the conclusion that pre-optimization > pushes the system away from the minimum energy path (leading to the > increased barriers)
This is indeed the expected behavior. If your first and last images were potential energy minimum configurations, even if local ones, the pre-optimization 'relax' calculations would let them unchanged, or almost unchanged because only in the case of high symmetry systems input coordinates are "exactly" in a local minimum. This implies that your first and last images are not potential energy minimum configurations, and the code correctly optimizes them, and this of course lowers the potential energy and raises the potential energy barrier connecting the two configurations. > In my opinion it is more physical if a migration corresponds to a > minimum energy pathway AND minimum restructuring. I'm not sure I understand this sentence. If you don't start from/end to a local minimum why do you gain more physical insight into your NEB simulation? If you want to drive your system through some intermediate local minimum which permits the lowering of the global barriers across the PES, you can use the intermediate_image feature. If in the end you find the same high barriers then three interpretations can be given: 1) your initial physical guess is wrong and in the "real" system vacancies do not exist/do not migrate; 2) The DFT level of theory used to perform the NEB calculation is not accurate and does not provide reliable numbers for transition states/barriers; 3) the migration occur through some redox process in which there is a change in the charge state of the defect which lowers the barrier. You are the only one that can think through this... HTH Giuseppe Quoting Aleksandra Oranskaia <aleksandra.oransk...@kaust.edu.sa>: > Hello dear QE users, > > I have a question regarding pre-optimization of starting and ending > points of NEB. > > Dealing with multi-scale simulation of a vacancy migration in a huge > supercell (say, 350-400 atoms), should one make a pre-optimization > of starting and ending points of a path? > > I tried both ways and came to the conclusion that pre-optimization > pushes the system away from the minimum energy path (leading to the > increased barriers), because pre-optimization implies too deep > restructuring towards deeper minimums rather than the local minimums > closest to the minimum energy pathway. > In my opinion it is more physical if a migration corresponds to a > minimum energy pathway AND minimum restructuring. > > If you would simulate vacancy migration in a huge supercell with NEB > optimization, would you optimize starting and ending points? Why? > > > Thanks in advance and hope to hear your opinions and reasonings, > > Regards, > Alex. > -- > > ------------------------------ > This message and its contents, including attachments are intended solely > for the original recipient. If you are not the intended recipient or have > received this message in error, please notify me immediately and delete > this message from your computer system. Any unauthorized use or > distribution is prohibited. Please consider the environment before printing > this email. > > _______________________________________________ > Pw_forum mailing list > Pw_forum@pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum -- ******************************************************** - Article premier - Les hommes naissent et demeurent libres et égaux en droits. Les distinctions sociales ne peuvent être fondées que sur l'utilité commune - Article 2 - Le but de toute association politique est la conservation des droits naturels et imprescriptibles de l'homme. Ces droits sont la liberté, la propriété, la sûreté et la résistance à l'oppression. ******************************************************** Giuseppe Mattioli CNR - ISTITUTO DI STRUTTURA DELLA MATERIA v. Salaria Km 29,300 - C.P. 10 I 00015 - Monterotondo Stazione (RM) Tel + 39 06 90672836 - Fax +39 06 90672316 E-mail: <giuseppe.matti...@ism.cnr.it> http://www.ism.cnr.it/en/staff/giuseppe-mattioli/ ResearcherID: F-6308-2012 _______________________________________________ Pw_forum mailing list Pw_forum@pwscf.org http://pwscf.org/mailman/listinfo/pw_forum