[Wien] Simulating charged defect

[E.A.Moore]

The defect energies for MgO and ZnO have been calculated many times, particularly by Prof. C. R. A. Catlow's group. I assume you have checked the literature.
If you seriously want just the defect energy for an isolated defect such as a Zn vacancy, periodic methods are not necessarily the best way to do this. There is a free program, GULP, provided by Prof. Julian Gale at Curtin University, Australia which uses a non-ab initio method but can actually calculate the energy of removing just one ion from a solid without using supercells.
If you want electronic or magnetic properties, you will of course need an ab initio program. 
For isolated defects using a supercell approach you do need of course to try increasingly larger supercells until you are sure that defects in adjacent cells are non-interacting.
Charged defects are not a good idea. The charge in one supercell is -2 but you have an infinite array of cells. Some programs allow you to add background charges to get over this problem. There is a WIEN2k FAQ on how to handle charged cells in WIEN2k. See the WIEN2k website.
The other consideration is that generally solids are not charged so that any defect has to be balanced by others to give overall electrical neutrality. For example for every Zn vacancy there could be an O vacancy. These are not necessarily close so that if you are interested in isolated defects, you do need to look at the separate defect energies. If however you have reason to believe your defects are clustered (for example a Zn vacancy balanced by a nearby Zn interstitial) then you can construct a supercell containing the charge-neutral cluster.
 
 
Elaine A. Moore
Department of Chemistry and Analytical Sciences
The Open University
UK
 
 
 
 
 
 

________________________________

From: wien-bounces at zeus.theochem.tuwien.ac.at on behalf of suman mandal
Sent: Thu 01/01/2009 12:02
To: wien at zeus.theochem.tuwien.ac.at
Subject: [Wien] Simulating charged defect



 Dear Prof. Blaha and WIEN2k users,

I am new with this code, having a basic idea. I have started a problem
for that I need to calculate the defect formation energy for oxide
materials (like ZnO, MgO etc. ). I understand that by removing one
atom from the structure file, one can simulate the vacancy with
supercell. But i really do not understand how to simulate a vacancy in
a charged state ( say -2 fo Zn vac) in supercell method. please help
me.
thanks in advance,

Suman Mandal
Saha Institute of Nuclear Physics.
Kolkata, India.
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