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# LOPTICS

LOPTICS = .TRUE. | .FALSE.
Default: LOPTICS = .FALSE.

Description: LOPTICS=.TRUE. calculates the frequency dependent dielectric matrix after the electronic ground state has been determined.

The imaginary part is determined by a summation over empty states using the equation:

$\epsilon _{{\alpha \beta }}^{{(2)}}\left(\omega \right)={\frac {4\pi ^{2}e^{2}}{\Omega }}{\mathrm {lim}}_{{q\rightarrow 0}}{\frac {1}{q^{2}}}\sum _{{c,v,{\mathbf {k}}}}2w_{{\mathbf {k}}}\delta (\epsilon _{{c{\mathbf {k}}}}-\epsilon _{{v{\mathbf {k}}}}-\omega )\times \langle u_{{c{\mathbf {k}}+{\mathbf {e}}_{\alpha }q}}|u_{{v{\mathbf {k}}}}\rangle \langle u_{{c{\mathbf {k}}+{\mathbf {e}}_{\beta }q}}|u_{{v{\mathbf {k}}}}\rangle ^{*}$ here the indices c and v refer to conduction and valence band states respectively, and uck is the cell periodic part of the orbitals at the k-point k. The real part of the dielectric tensor ε(1) is obtained by the usual Kramers-Kronig transformation

$\epsilon _{\alpha \beta }^{(1)}(\omega )=1+{\frac {2}{\pi }}P\int _{0}^{\infty }{\frac {\epsilon _{\alpha \beta }^{(2)}(\omega ')\omega '}{\omega '^{2}-\omega ^{2}+i\eta }}d\omega '$ where P denotes the principle value. The method is explained in detail in the paper by Gajdoš et al. (see Eqs. 15, 29, and 30). The complex shift η is determined by the parameter CSHIFT.

Note that local field effects, i.e. changes of the cell periodic part of the potential are neglected in this approximation. These can be evaluated using either the implemented density functional perturbation theory (LEPSILON=.TRUE.}}), or the GW routines. The method selected using LOPTICS=.TRUE. requires an appreciable number of empty conduction band states. Reasonable results are usually only obtained, if the parameter NBANDS is roughly doubled or tripled in the INCAR file with respect to the VASP default. Furthermore it is emphasized that the routine works properly even for HF and screened exchange type calculations and hybrid functionals. In this case, finite differences are used to determine the derivatives of the Hamiltonian with respect to k.

Note that the number of frequency grid points is determined by the parameter NEDOS. In many cases it is desirable to increase this parameter significantly from its default value. Values around NEDOS=2000 are strongly recommended.