[Pw_forum] Supercell size in phonon calculations

[Charlie Ruffman]

Hi Stefano and others,


Thank you for your helpful response!


We are fairly new to both QE and phonon calculations, and have been trying to simulate the Raman spectrum of an extended 2D sheet using the primitive cell and also a 3x3 supercell. A phonon calculation at q=0 of the primitive cell produces results that are very close to experiment (at 16x16x1 Monkhorst pack k-points, 1e-8 Ry for electronic convergence, and a kinetic energy cutoff of 50 Ry). However, calculation of the 3x3 supercell at q=0 (using the same convergence criteria and 8x8x1 k-points) produces additional Raman active modes, as well as shifting some of the modes present in the primitive cell to different energies. If the additional Raman active modes in the supercell are from  q/=0 in the primitive cell, why might these be Raman active? Also, is it possible to reliably simulate the Raman spectrum of a 3x3 or larger supercell?


Thank you again for your help with this.


--
Charlie Ruffman | BSc
BSc(Hons) Student

Chemistry Department
University of Otago
Tel 64 3 479 7930
PO Box 56, Dunedin 9054
New Zealand

Website http://neon.otago.ac.nz/research/alg/
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