[QE-users] vc-relax for orthorhombic systems

[Lorenzo Paulatto]

> actually requires, etc), continue with vc-relax using dofree = ibrav 

Hello Aleksandra, I've implemented that constraint in a very simple way, 
as long as the final pressure/strain is zero, there should be no 
problem, but better use it with a grain of salt.

Also, keep in mind that vc-relax is done at fixed number of plane-waves, 
not constant cutoff. You should always repeat it starting from the final 
configuration and


> 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 :-(

Do you have any reason to expect that DFT should perform better than 
this? It does not look like a very big discrepancy. 0.1 A is the order 
of the difference between PBE GGA and PZ LDA for very simple materials. 
Correcting for the lack of VdW interaction with a simple Grimme-D2 
method could be enough to get a better agreement.

I did some calculations in orthorhombic cells myself, and found it very 
difficult to be completely sure about the final relaxation. Because the 
material can be sort of soft, it is possible that a deformation that 
changes the angles and the cell sides while keeping the volume about 
constants has a very small energy gradient.

I would also guess that thermal effects could be important, hence 
considering the Free Energy (Enthalpy - TS) could be necessary. 
Unfortunately, this is quite expensive and complicated, even at the 
Quasi-Harmonic level.



> I would hugely appreciate any comments,
> 
> 
> Thanks,
> Alex.
> ___
> Aleksandra Oranskaia (M.Sc.)
> ChemS PhD student, KAUST
> Phone: +966 50 1335254
> 
> 
> 
> 
> 
> 
> 
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-- 
Lorenzo Paulatto - Paris