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

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

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 Sections

LOPTICS, CSHIFT

References

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