1 of 9

Structural Properties�with an excursion to XC functional

Abdul Muhaymin

Graduate student, MSN, Bilkent University

2 of 9

Outline

Material Structure

We need a CIF file or a POSCAR file or an XSF or some type file format where the crystal structure is saved.
CIF files are kind of universal. Use materials-project for this.
If your material's geometry is unknown (i.e. you don't even know the crystal structure, let alone atomic positions), try USPEX. �https://uspex-team.org/en/uspex/overview
You can find stable structure by relaxation if you know a nearby structure.
For doping/defect, change the atomic labels/position manually in the CIF/POSCAR/input file.
ALWAYS visualize your structure before running calculation!

Convergence testing

Why do we need convergence? - consistency
Millions of papers, mostly garbage!
Do convergence test for everything!
Changed a single parameter? Rerun the convergence test. No mercy!
Usual workflow: relax the structure, kinetic energy cutoff convergence, k-point convergence, smearing convergence, supercell size convergence
Convergence = energy converged within a certain threshold per atom
Strongly depends on the pseudopotential
Literature is simply a guide, don't take values from them

Ionic relaxation

Different from the inner SCF loop
Outer loop can be implemented by various algorithms, 'bfgs' being the most popular
Note that here the structure is not changing (supercell shape remains intact)
Only atomic positions are changing

But how do we get the force?

Answer: The Hellmann-Feynman theorem which tells us that forces are the expectation value of the derivative of the external potential only.

In practice, existing codes usually use something different!
VASP users can use machine-learned force-field (check it out if you are working with large supercell)

Cell relaxation

You can change cell angle, cell side length (and thus cell shape)
In QE, calculation='relax' is for relaxation calculation, calculation='vc-relax' is for cell relaxation.
In VASP, use the appropriate ISIF tag to control the DOF. Structural optimization cannot ensure global minima
Plausibility of metastable structure
Note the two extra namelists:�&IONS�/�&CELL�/

Strained system

Change the lattice parameter to induce uniform pressure
Strain can open/close band gap
'band-gap engineering' or 'strain engineering' - hot topic of research, especially in the field of wide band gap semiconductor and novel semiconductors
Figure is of band structures of 2D MoS2 film under strain

XC Functional