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LNABLA: transversal gauge



Usually VASP uses the longitudinal expression for the frequency dependent dielectric matrix as described in the preceeding section (see. 6.72.1). It is however possible to switch to the computationally somewhat simpler transversal expressions by selecting LNABLA=.TRUE. (in this case Eq. (17) and (20) in Ref. [108]). In this simplification the imaginary part of the macroscopic dielectric function $ \varepsilon_{\infty}^{(2)}$ is given by

$\displaystyle \varepsilon^{(2)}_{\alpha \beta} (\omega) = \frac{4 \pi^2 e^2 \hb...{\nabla}}_{\beta} - {\mathbf{ k}}}_{\beta} \vert u_{v{\mathbf{k}}} \rangle^*.$ (6.77)

Except for the purpose of testing, there is however hardly ever a reason to use the transversal expression, since it is less accurate.[108]

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