Lattice Dynamics

Some Theory of Lattice Dynamics

An explanation of lattice dynamics from Stewart Clark's dissertation, Lattice Dynamics, strategies for parrallelization in [3]

Dove, M. T. "Introduction to the theory of lattice dynamics." École thématique de la Société Française de la Neutronique. 12 (2011): 123-159. Link.
Debye. Crystal Lattice Dynamics. Link
Clark, Stewart J., Graeme J. Ackland, and Jason Crain. "Theoretical study of high-density phases of covalent semiconductors. II. Empirical treatment." Physical Review B 49.8 (1994): 5341. Link

Sample a line of kpoints from the points of high-symmetry. Curtorolo provides some sample pathways in which to build dispersion curves[1]

Setyawan, Wahyu, and Stefano Curtarolo. "High-throughput electronic band structure calculations: Challenges and tools." Computational materials science 49.2 (2010): 299-312. Link

http://www.crystal.unito.it/mssc2016/Lectures/QHA.pdf

CALCULATING PHONONS IN VASP

Technical Problems
- (a) small supercell size.
- (b) stopping criterion for ionic relaxation is not tight enough (EDIFFG ~ -0.001 recommended).
- (c) forces are not accurate and EDIFF needs to be tightened (EDIFF ~ 1E-8 , ADDGRID=.TRUE., and LREAL =. FALSE. are recommended).
- (d) The DFT functional "loosens" the lattice a bit. This is typical for GGA family.
- (e) many other things one can think of
Physical Problems
- (a) The crystal is unstable at 0 K and the imaginary frequency indicates a phase transition and the stability of another phase. A classic example is cubic ZrO2.
- (b) The crystal is stable at 0K but is highly anharmonic and a harmonic treatment will artificially show imaginary frequency.
- (c) Other suggestions from phonon experts ...?

[1] https://sourceforge.net/p/phonopy/mailman/message/34833680/

CALCULATING PHONONS IN GULP

[1] GULP Manual (link)

CALCULATING PHONONS USING PHONOPY

http://people.bath.ac.uk/aw558/presentations/phonopy_tips_2015.pdf

Complete documentation for PhonTS can be found in Ref [1] and background is in Ref[2]. I am currently writing a python interface to PhonTS.

Calculating Phonon Dispersion Curve:

[1] Phonon Transport Simulator (PhonTS)

[2] Chernatynskiy, Aleksandr, and Simon R. Phillpot. "Phonon transport simulator (PhonTS)." Computer Physics Communications 192 (2015): 196-204.Link

- Dispersion Curve

PHONON CALCULATION COOKBOOK FOR VASP:

Google Sites

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