Supercell calculations on TiC

by **Algerien1970** Fri 29 May - 16:06

From the userguide :

This example shows you how to create a supercell of TiC, which could be used to simulate a TiC-surface or vacancies, impurities or core-holes for X-ray absorption / ELNES spectroscopy. I'll describe the procedure using Unix and WIEN2k commands in an xterm, but of course you can do the same in w2web.
Create a new directory, copy the original TiC struct file into it and run supercell program:

mkdir super
cd super
cp ../TiC/TiC.struct .
x supercell

Specify ``TiC.struct'', a ``2x2x2'' supercell, ``F'' lattice (this will create a cell with 16 atoms, you can also create 32 or 64 atom cells using B or P lattice type. Note: surfaces require a P supercell).
cp TiC_super.struct super.struct

and edit this file to make some changes. You could eg.

delete an atom (to simulate a vacancy)
replace an atom by another element (impurity)
``label'' an atom (put a 1 in the 3rd column next to the element name) to make this atom unique (needed eg. for core-holes)
displace an atom (for phase transitions or phonons)

Note: it is important to make at least one of these chages. Otherwise the initialization will restore the original unit cell (or the calculations will fail later on because symmetry is most likely not correct)

Run init_lapw. You will see that nn complains and finds a new set of equivalent atoms (originally all atoms were non-equivalent, but nn finds that some atoms have identical neighbors, thus should be in an equivalent set). Accept the automatically generated struct file and continue. Remember, supercells normally require less k-points than the original small cell.

After the complete initialization you may in principle restore the original struct file (eg. without a displacement) in case you want to ``repeat'' the undistorted structure in supercell geometry.

For a ``core-hole'' calculation you would now edit super.inc and remove one core electron from the desired atom and state (1s or 2p, ...). In addition you should add the missing electron either in super.inm(background charge) or super.in2 (add it to the valence electrons). In the latter case, you should remove this extra electron AFTER scf and BEFORE calculation of the spectra.

Once this has been done, you could start a scf-cycle (for impurities, vacancies,.. you should most likely also optimize the internal positions).