LNABLA

LNABLA = .TRUE. | .FALSE.
Default: LNABLA = .FALSE. 

Description: LNABLA=.TRUE. evaluates the transversal expression for the frequency dependent dielectric matrix.

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Usually VASP uses the longitudinal expression for the frequency dependent dielectric matrix. It is however possible to switch to the computationally somewhat simpler transversal expressions by selecting LNABLA=.TRUE. (Eqs. 17 and 20 in Ref.). In this simplification the imaginary part of the macroscopic dielectric function is given by

$\epsilon^{(2)}_{\alpha \beta} (\omega) = \frac{4 \pi^2 e^2 \hbar^4}{\Omega \omega^2 m_e^2} \mathrm{lim}_{\mathbf{q} \rightarrow 0} \sum_{c,v, \mathbf{k}} 2 w_\mathbf{k} \delta( \epsilon_{c\mathbf{k+q}} - \epsilon_{v\mathbf{k}} - \omega) \times \langle u_{c\mathbf{k}} | i{\mathbf{\nabla}_{\alpha} - \mathbf{k}}_{\alpha} | u_{v\mathbf{k}} \rangle \langle u_{c\mathbf{k}} | i{\mathbf{\nabla}_{\beta} - \mathbf{k}}_{\beta} | u_{v\mathbf{k}} \rangle^*.$

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

Related tags and articles

LOPTICS, CSHIFT

Examples that use this tag

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