[QE-users] vc-relax for orthorhombic systems

[Aleksandra Oranskaia]

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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