I would like to know how to choose the correct value of nk1, nk2, nk3 ? in DOS file while doing calculations for phonon density of states in QUANTUM ESPRESSO for any material in general and particularly for polar materials.
For phonon DOS calculations, you can usually use large values. Just make sure they are dividable by the nk1, nk2 and nk3 chosen in your SCF step. For example, if you have calculated the wave functions using a 12x12x12 k-mesh then it would be better to use a 24x24x24 q-mesh for the phonon DOS calculation. In case, your BZ has an elongated shape, you also need to make sure the q-mesh chosen produces comparable increments along all crystalline directions.
This may help too: https://www.researchgate.net/post/How_to_obtain_Phonon_dispersion_for_ZnS_in_Quantum_espresso
we have to choose nk1,nk2,nk3 in such a way kmesh grid taken in SCF calculations is multiple of the same.
Also you can check for the convergence of the properties wrt to same to ensure the vailidiy of results.
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