[QE-users] vc-relax for orthorhombic systems

[Giuseppe Mattioli]

Dear Alex
Only a comment: Why do you need to be so tightly close to experimental  
lattice parameters? They are generally not the final output of a bunch  
of simulations. As you are not trying to validate a new functional or  
some new correction, your results seem to be close enough to  
experimental data to ensure the calculation of further parameters  
(DOS, band structure, absorption spectroscopy, conductivity, ...). If  
you need *exactly* to work with experimental lattice parameters for  
some reason instead, then use experimental values and do not perform  
vc-relax but relax calculations. Otherwise, I would start asking which  
experimental parameters are you trying to reproduce, e.g., at which  
temperature, how many defects were present in the experimental sample,  
... And I would answer that a "true experimental value" probably does  
not exist...

As you need practical answers and not philosophical ones, I would be  
satisfied of a result within 1% from "some measurements"

HTH
Giuseppe


Aleksandra Oranskaia <aleksandra.oranskaia at kaust.edu.sa> ha scritto:

> 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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>> ----------------------------------------------------------------------
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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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GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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