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| | | #Redirect [[Time-evolution_algorithm]] |
| Description: {{TAG|ALGO}}= timeev calculates the frequency dependent dielectric matrix after the electronic ground state has been determined using the time evolution algorithm (only available in vasp.6)
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| ----
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| The timepropagation algorithm applies a short delta puls (E field) in time, and
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| then follows the evolution of the dipole moments. The Green-Kubo relation
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| allows to calculate the frequency dependent dielectric response function
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| from the time evolution of the dipole moments <ref name="kubo:57"/>.
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| Details of the implementation are explained in Ref. <ref name="sander:prb:2015"/>. The
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| time propagation algorithm in VASP uses relatively large time steps by projecting,
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| after each time step, onto a specific number of occupied and unoccupied pais. The number of
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| occupied and unoccupied pairs are controlled by the tags {{TAG|NBANDSO}} and {{TAG|NBANDSV}}
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| and {{TAG|OMEGAMAX}} -
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| in the same manner as done for Casida and [[BSE calculations]].
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| This has the advantage that the results are strictly compatible to the results
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| obtained by the [[BSE calculations]].
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| The disadvantage is that a sufficient number of unoccupied orbitals need to
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| be calculated in the preceding ground state calculations
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| (note however, that unoccupied orbitals are not propagated in time, which
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| saves compute time).
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| Per default, the time propagation code includes Hartree and local field
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| effects ({{TAG|LHARTREE}}=.TRUE. and {{TAG|LFXC}}=.TRUE.). Results in the independent particle approximation can be calculated by setting {{TAG|LHARTREE}}=.FALSE. and {{TAG|LFXC}}=.FALSE.
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| Other combinations ({{TAG|LHARTREE}}=.TRUE. and {{TAG|LFXC}}=.FALSE. or
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| {{TAG|LHARTREE}}=.FALSE. and {{TAG|LFXC}}=.TRUE. are presently not supported).
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| The number of timesteps performed in the propagation is usually inverse proportional
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| to the value of {{TAG|CSHIFT}}. That is a small {{TAG|CSHIFT}} will require
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| less time step (but yield a more strongly broadened spectrum), whereas
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| a small shift {{TAG|CSHIFT}} will require more time steps.
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| Typical values of around {{TAG|CSHIFT}}=0.1 will result in useful spectra.
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| Alternatively, the number of time steps can be set directly by the tag {{TAG|NELM}}.
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| In this case, the number of user supplied steps needs to exceed {{TAG|NELM}}>100 (otherwise, the value
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| in NELM will be disregarded, and the number of time steps is determined by
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| the tag {{TAG|CSHIFT}}.
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| Finally, the tag {{TAG | IEPSILON}} controls the Cartesian direction along which
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| the delta pulse is applied. {{TAG | IEPSILON}}=4 (default) performs
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| three independent calculations for an electric field in x, y and z direction
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| (and is therefore most expensive).
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| VASP posses multiple other routines to calculate the frequency dependent dielectric function.
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| The simplest approach uses the independent particle approximation ({{TAG|LOPTICS}}=.TRUE).
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| Furthermore, one can use {{TAG|ALGO}} = TDHF ([[BSE calculations]] equivalent to solving the Casida equation), {{TAG|ALGO}} = GW ([[GW calculations]]).
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| For standard DFT, the timeevolution algorithm is
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| usually fastest, whereas for hybrid functionals {{TAG|ALGO}} = TDHF is
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| usually faster. Results of timeevolution are strictly identical to
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| {{TAG|ALGO}} = TDHF; {{TAG|ANTIRES}} = 2, if the tags {{TAG|CSHIFT}}, {{TAG|OMEGAMAX}}
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| {{TAG|NBANDSV}}, and {{TAG|NBANDSO}} are chosen identical
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| ({{TAG|ANTIRES}} = 2 is required, since time propagation does not rely on
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| the Tamm Dancoff approximation).
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| == Related Tags and Sections ==
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| {{TAG|CSHIFT}},
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| {{TAG|LHARTREE}},
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| {{TAG|LFXC}},
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| {{TAG|NBANDSV}},
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| {{TAG|NBANDSO}},
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| {{TAG|OMEGAMAX}}
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| see also [[BSE calculations]]
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| == References ==
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| <references>
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| <ref name="kubo:57">[http://journals.jps.jp/doi/10.1143/JPSJ.12.570 R. Kubo, Statistical-Mechanical Theory of Irreversible Processes. I. General Theory and Simple Applications to Magnetic and Conduction Problems. In: Journal of the Physical Society of Japan. Band 12, Nr.6, 15. Juni 1957, S.570–586, doi:10.1143/JPSJ.12.57].</ref>
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| <ref name="sander:prb:2015">[https://journals.aps.org/prb/abstract/10.1103/PhysRevB.92.045209 T. Sander, E. Maggio, and G. Kresse, Beyond the Tamm-Dancoff approximation for extended systems using exact diagonalization. Physical Review B, 92, 045209 (2015).]
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| </ref>
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| </references>
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| ----
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| [[The_VASP_Manual|Contents]]
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| [[Category:INCAR]][[Category:Linear response]]
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