[Pw_forum] relax of 216 atoms (cubic structure)

[Yevgen Melikhov]

Dear all users of QE, I am trying to relax GaAs doped with alittle bit of Mn and with some point defects and, of course, I have someproblems in doing that. In order to construct my initial structureI took cubic cell GaAs with 8 atoms and translated it: (I) 3x2x2 to produce a structurewith 96 atoms and (II) 3x3x3 to produce a structure with 216 atoms. After thatI substitute Ga with Mn (where necessary) and introduce point defects (wherenecessary).  The structure with 96 atoms (sometimes 94or 95 depending on the problem) with GAMMA point relaxes quite well (please seethe attached input file).  However, I could not relax the structurewith 216 atoms (sometimes 214 or 215) (please see the attached input and output files).  If I choose GAMMA point only (that is, ofcourse, what I wanted to speed up calculations), the error is:%%%%%%%%%%%%%%%%%%%%%%%Error in routine efermig (1):Internal error, cannot bracket Ef %%%%%%%%%%%%%%%%%%%%%%%which usually happens in the SCF iteration#2 of the first BFGS cycle.After checking the forum and playing withthe parameters I decided to increase the number of k-point.  If I choose more k-points (as in the attachedfile), the SCF cannot converge (usually this happens within the second orsometimes third BFGS cycle) for 70-100 cycles and, due to time limit on thecluster, the job is not finished. I tried to restart the job but the problem,in my opinion, is not in having not enough time to finish SCF calculations but ratherin oscillations of the scf accuracy (please see the extracted parts of the output file). The first BFGS cycle usually is convergedwithin 10-20 SCF cycles. Is my assumption that I need to changeconv_thr to lager values correct: the number of atoms doubled (96 vs216), so is the energy (-10300 Ry vs -23000 Ry), and therefore, at least I needto double the conv_thr (currently it is at its default value 1.0D-6)? For sucha big system may I even increase it further 1.0D-4? Thank you in advance for anyhints/comments! Yevgen Melikhov Lab of X-ray and Electron MicroscopyResearchInstitute of Physics, PASWarsaw, Poland
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