# Comparing phonon dispersions generated with phonopy to those generated with Quantum Espresso

I've generated a phonon dispersion for BCC lithium using the python api for phonopy. I specified the following q-point path

path = [[[0.0, 0, 1.0], [0.5, 0.5, 0.5],[0, 0, 0], [0.0, 0.25, 0.25]]]


and then run

phonon.run_band_structure(qpoints, path_connections=connections, labels=labels)


After using the utility

phonopy-bandplot  --gnuplot band.yaml


I obtain a .plot file for comparison with a QE phonon dispersion. I noticed that I had to scale the phonopy q-vector axis a factor equal to the lattice paramter to match the two up. However, even doing this, I find that the phonopy dispersion abruptly ends before the final q-point segment has completed. I wondered if anyone has tried to do this before and knows how to correctly compare the two?

P.S. The q-point path I specify in QE is

   5
0.00000000 0.00000000 0.0   40
0.0 0.0 1.0   40
0.5 0.5 0.5   40
0.00000000 0.00000000 0.0   40
0.0        0.5    0.5   1


Directly translating this into the phonopy path

path = [[[0, 0, 0], [0.0, 0, 1.0], [0.5, 0.5, 0.5],[0, 0, 0], [0.0, 0.5, 0.5]]]


gives totally different results (as if extra segments have been added to the path). I think I'm misunderstanding how phonopy creates its band paths.

• Actually, I think I solved the problem. I had to use the path path = [[[0, 0, 0], [-0.5, 0.5, 0.5], [0.25, 0.25, 0.25],[0, 0, 0], [0.0, 0.5, 0.0]]] even although I specified that my lattice vectors should be [[-0.5,0.5,0.5],[0.5,-0.5,0.5],[0.5,0.5,-0.5]] (the path would ostensibly correspond to the choice of lattice vectors [[0.5,0.5,-0.5],[-0.5,0.5,0.5],[0.5,-0.5,0.5]] but maybe phonopy does something under the hood that I'm not aware of) . Seems strange but figured I'd leave this here in case it's helpful to anyone else. Commented Oct 29, 2023 at 9:08