[QE-users] vc-relax for orthorhombic systems

[Aleksandra Oranskaia]

Right now I am testing some desperate approaches like increasing k-points, decreasing mixing_beta together with trust_radii to force systems to change sightly fro step to step in vc-relax, etc.

I also thought that more precise calculation of stresses (in vc-relx routine) might help, does anyone have experience with parameters involved in stress calculations like ecfixed, qcutz, q2sigma?



Best,
Alex.
___
Aleksandra Oranskaia (M.Sc.)
ChemS PhD student, KAUST
Phone: +966 50 1335254






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>   1. vc-relax for orthorhombic systems (Aleksandra Oranskaia)
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> Message: 1
> Date: Fri, 5 Apr 2019 19:39:39 +0300
> From: Aleksandra Oranskaia <aleksandra.oranskaia at kaust.edu.sa>
> To: Quantum Espresso users Forum <users at lists.quantum-espresso.org>
> Subject: [QE-users] vc-relax for orthorhombic systems
> Message-ID: <8FCA3967-9817-4748-B8D3-34EC6F0295A2 at kaust.edu.sa>
> Content-Type: text/plain; charset="iso-8859-1"
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> Dear QE users and developers,
> 
> 1 When dealing with orthorhombic systems, I start with space_group super-tight relaxation of the atomic positions (keeping right balance in force and scf accuracy, increasing ecutrho plus 100 Ry than my system actually requires, etc), continue with vc-relax using dofree = ibrav (and press_thr = 0.0), and Always obtain at least one lattice constant screwed up (with an error like +/- 0.1 A as compared to the experimental constant).
> I tried 1000 setups for a few systems (varying functionals, vdw corrections, different pseudo libraries, etc) and have a feeling that sth is not right with vc-relax: for other than orthorhombic systems the results match the experimental lattice constants with an acceptable error.
> 
> 2 I was wondering how vc-relax work for big orthorhombic unit cells. I took alpha-S8 (space group 70, so 128 atoms turn into 32 within ibrav = 10 with 8 symmetry elements) and am not satisfied with any lattice constants: on PBEsol level, error in volume is <1% as compared to the experimental unit cell, but lattice constants are like 10.602 (exp: 10.465), 12.937 (exp: 12.866), 24.286 (exp: 24.486) A that do not look acceptable at all :-(
> 
> I would hugely appreciate any comments,
> 
> 
> Thanks,
> Alex.
> ___
> Aleksandra Oranskaia (M.Sc.)
> ChemS PhD student, KAUST
> Phone: +966 50 1335254
> 
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