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When I calculate the ionic contributions to the frequency dependent dielectric function using DFPT, the frequency range is too small to get the frequency of LO branch phonons. Is there any parameter to adjust the frequency range?

Dear Jun-Jie Shi,

The frequency range of the ionic contribution to the dielectric function directly results from the range of phonon frequencies. In other words, if you see the LO branch in the phonon spectrum, then it should also be accounted for in the ionic contribution to the dielectric function.

The size of the Hessian matrix determines the number of phonon modes. With the current implementation in VASP, this depends on the size of the supercell for both methods, finite differences and density-functional-perturbation theory (DFPT). Therefore, please try to set up a larger supercell, in order to increase the range of frequencies in your calculation.

Best regards,
Marie-Therese