Re: [Pw_forum] Supercell size in phonon calculations
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/ ___ Pw_forum mailing list Pw_forum@pwscf.org http://pwscf.org/mailman/listinfo/pw_forum
[Pw_forum] Supercell size in phonon calculations
Hi all, I have recently become interested in how the size of the unit cell in phonon calculations effects the results produced by the calculation (Raman spectra in particular). It seems that increasing the number of atoms explicitly simulated should also increase the number of normal modes calcuated? Does anyone know if using the primitive cell will produce more or less accurate results than a larger supercell? Thanks -- 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/ ___ Pw_forum mailing list Pw_forum@pwscf.org http://pwscf.org/mailman/listinfo/pw_forum
Re: [Pw_forum] Acoustic sum rule in 2D surfaces
Hi, Thank you for your very helpful and detailed response Lorenzo. We have now been able to produce nice Raman spectra for our system. On a related topic, we were interested in how the choice of supercell size might influence the vibrational modes we calculate. For instance, looking at a the smallest repeatable unit of our system naturally yields fewer vibrational modes than simulating a cell of 2 or 3 times the size. We were wondering whether allowing the extra freedom associated with a larger cell might actually map worse onto experimental results? Thanks again, Charlie Ruffman University of Otago New Zealand ___ Pw_forum mailing list Pw_forum@pwscf.org http://pwscf.org/mailman/listinfo/pw_forum
[Pw_forum] Acoustic sum rule in 2D surfaces
Hi all, My group has recently begun using QE to simulate the Raman spectra of 2D bulk MoS2 surfaces. The surface is periodic in the x and y direction. We typically calculate at least one imaginary mode of vibration (at ~ 34 cm-1), and were wondering if it was valid to apply a "crystal" acoustic sum rule, which removes the imaginary frequency. Also, does anyone have insight into whether we should be applying a different type of acoustic sum rule than crystal (eg. one-dim or zero-dim)? Thanks for your time, Charlie Ruffman University of Otago, New Zealand ___ Pw_forum mailing list Pw_forum@pwscf.org http://pwscf.org/mailman/listinfo/pw_forum
