[QE-users] gradients with charge compensation

[Andreussi, Oliviero]

Dear Alex,

As Paolo replied, jellium is unavoidable in periodic simulations of charged systems. If you are using periodic conditions to study an isolated system or other partially periodic systems, then there are ways to avoid the jellium and correct for the pbc artifacts. In your molecular cation optimization, you can have results equivalent to free boundary conditions if you specify assume_isolated = ‘martyna-tuckermann’ and choose a cell size twice as large as your cation. If you plan to study charged defects in periodic structures (crystals, liquids, amorphous), then there are very few things you can do apart from increasing the cell size.

Calculations on cations in QE do work fine, forces are usually ok and geometry optimizations converge, unless there are other physical/numerical problems in your input. Mind that anions, instead, may pose convergence problems in QE due to the well-known problems of DFT to localize the extra electron. This issue is not seen in localized basis set simulations, as the electron has no way to delocalize, but in plane-wave setups calculations on anions may not converge, unless one uses some tricks.

What do you mean with the sentence “without jellium the optimized structure is very close to the real one”? Without jellium means for a charge neutral molecule? The real one means the one of Gaussian?

Oliviero Andreussi
--
Assistant Professor
Department of Physics
University of North Texas
Email: oliviero.andreussi at unt.edu<mailto:oliviero.andreussi at unt.edu>
Skype: olivieroandreussi
Web: https://sites.google.com/site/olivieroandreussi

On Apr 25, 2018, at 6:03 AM, Aleksandra Oranskaia <aleksandra.oranskaia at kaust.edu.sa<mailto:aleksandra.oranskaia at kaust.edu.sa>> wrote:

Hi dear users and developers of QE,

If one is interested in studying charged state defects -- is it correct to run supercell optimizations with a compensating jellium background ?
Let’s say, if one is interested in +1 point defect state -- is it correct to add tot_charge = +1 ?

I quickly checked if the gradients for molecular cation optimization are reasonable with such an approach. For this I took small cation with known structure and:
- gaussian09-d.01 optimization with 6-11G**/B3LYP, +1 charge, 1 spin multiplicity gave perfectly matching structure with the real one
- QE-6.2.1 optimization with vdw-df2/USPP and tot_charge = +1 destroyed the cation, it dissociated; without “jellium” the optimized structure is very close to the real one.

I would greatly appreciate your advice on how to obtain realistic structures of the supercells with charged defects,


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







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