|
|
| (17 intermediate revisions by 5 users not shown) |
| Line 1: |
Line 1: |
| | | #Redirect [[Time-evolution_algorithm]] |
| Description: {{TAG|ALGO}}= timeev calculates the frequency dependent dielectric function after the electronic ground state has been determined using the time evolution algorithm (only available in vasp.6)
| |
| ----
| |
| | |
| The timepropagation algorithm applies a short delta puls (E field) in time, and
| |
| then follows the evolution of the dipole moments. The Green-Kubo relation
| |
| allows to calculate the frequency dependent dielectric response function
| |
| from the time evolution of the dipole moments <ref name="kubo:57"/>.
| |
| | |
| Details of the implementation are explained in Ref. <ref name="sander:prb:2015"/>. The
| |
| time propagation algorithm in VASP uses relatively large time steps by projecting,
| |
| after each time step, onto a specific number of occupied and unoccupied pais. The number of
| |
| occupied and unoccupied pairs are controlled by the tags {{TAG|NBANDSO}} and {{TAG|NBANDSV}}
| |
| and {{TAG|OMEGAMAX}} -
| |
| in the same manner as done for Casida and [[BSE calculations]].
| |
| This has the advantage that the results are strictly compatible to the results
| |
| obtained by the [[BSE calculations]].
| |
| The disadvantage is that a sufficient number of unoccupied orbitals need to
| |
| be calculated in the preceding ground state calculations
| |
| (note however, that unoccupied orbitals are not propagated in time, which
| |
| saves compute time).
| |
| | |
| Per default, the time propagation code includes Hartree and local field
| |
| 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.
| |
| Other combinations ({{TAG|LHARTREE}}=.TRUE. and {{TAG|LFXC}}=.FALSE. or
| |
| {{TAG|LHARTREE}}=.FALSE. and {{TAG|LFXC}}=.TRUE. are presently not supported).
| |
| | |
| The number of timesteps performed in the propagation is usually inverse proportional
| |
| to the value of {{TAG|CSHIFT}}. That is a small {{TAG|CSHIFT}} will require
| |
| less time step (but yield a more strongly broadened spectrum), whereas
| |
| a small shift {{TAG|CSHIFT}} will require more time steps.
| |
| Typical values of around {{TAG|CSHIFT}}=0.1 will result in useful spectra.
| |
| Alternatively, the number of time steps can be set directly by the tag {{TAG|NELM}}.
| |
| In this case, the number of user supplied steps needs to exceed {{TAG|NELM}}>100 (otherwise, the value
| |
| in NELM will be disregarded, and the number of time steps is determined by
| |
| the tag {{TAG|CSHIFT}}.
| |
| | |
| Finally, the tag {{TAG | IEPSILON}} controls the Cartesian direction along which
| |
| the delta pulse is applied. {{TAG | IEPSILON}}=4 (default) performs
| |
| three independent calculations for an electric field in x, y and z direction
| |
| (and is therefore most expensive).
| |
| | |
| | |
| | |
| VASP posses multiple other routines to calculate the frequency dependent dielectric function.
| |
| The simplest approach uses the independent particle approximation ({{TAG|LOPTICS}}=.TRUE).
| |
| Furthermore, one can use {{TAG|ALGO}} = TDHF ([[BSE calculations]] equivalent to solving the Casida equation), {{TAG|ALGO}} = GW ([[GW calculations]]).
| |
| For standard DFT, the timeevolution algorithm is
| |
| usually fastest, whereas for hybrid functionals {{TAG|ALGO}} = TDHF is
| |
| usually faster. Results of timeevolution are strictly identical to
| |
| {{TAG|ALGO}} = TDHF; {{TAG|ANTIRES}} = 2, if the tags {{TAG|CSHIFT}}, {{TAG|OMEGAMAX}}
| |
| {{TAG|NBANDSV}}, and {{TAG|NBANDSO}} are chosen identical
| |
| ({{TAG|ANTIRES}} = 2 is required, since time propagation does not rely on
| |
| the Tamm Dancoff approximation).
| |
| | |
| == Example ==
| |
| | |
| A typical calculation does require two steps. First a groundstate
| |
| calculation using
| |
| | |
| {{TAGBL|System}} = Si
| |
| {{TAGBL|NBANDS}} = 12 ! even 8 bands suffice for Si
| |
| {{TAGBL|ISMEAR}} = 0 ; {{TAGBL|SIGMA}} = 0.05
| |
| {{TAGBL|ALGO}} = N
| |
| {{TAGBL|LOPTICS}} = .TRUE.
| |
| {{TAGBL|KPAR}} = 4 ! assuming we run on 4 cores, this will be fastest
| |
| | |
| | |
| And then a second calculation performing the actual time propagation:
| |
| | |
| {{TAGBL|System}} = Si
| |
| {{TAGBL|NBANDS}} = 12 ! even 8 bands suffice for Si
| |
| {{TAGBL|ISMEAR}} = 0 ; {{TAGBL|SIGMA}} = 0.05
| |
| {{TAGBL|ALGO}} = TIMEEV
| |
| {{TAGBL|IEPSILON}} = 1 ! cubic system, so response in x direction suffices
| |
| {{TAGBL|NBANDSO}} = 4 ; {{TAGBL|NBANDSV}} = 8 ; {{TAGBL|CSHIFT}} = 0.1
| |
| {{TAGBL|KPAR}} = 4 ! assuming we run on 4 cores, this will be fastest
| |
| | |
| In this case, {{TAG|OMEGAMAX}} is set automatically to the maximum transition energy
| |
| (in this example about 25 eV). Reducing the number of considered transitions, and thus
| |
| reducing OMEGAMAX, will increase the time step, and hence reduce the number of required time steps.
| |
| | |
|
| |
| For standard DFT calculations, the time propagation code is so fast that
| |
| very dense k-point grids can often be used.
| |
| | |
| == Related Tags and Sections ==
| |
| {{TAG|CSHIFT}},
| |
| {{TAG|LHARTREE}},
| |
| {{TAG|LFXC}},
| |
| {{TAG|NBANDSV}},
| |
| {{TAG|NBANDSO}},
| |
| {{TAG|OMEGAMAX}}
| |
| | |
| see also [[BSE calculations]]
| |
| | |
| == References ==
| |
| <references>
| |
| <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>
| |
| | |
| <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).]
| |
| | |
| </ref>
| |
| </references>
| |
| ----
| |
| [[The_VASP_Manual|Contents]]
| |
| | |
| [[Category:INCAR]][[Category:Linear response]]
| |
