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

Examples that use this tag

References

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