Time Evolution: Difference between revisions
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the tag {{TAG|CSHIFT}}. | the tag {{TAG|CSHIFT}}. | ||
Finally, the tag {{TAG | IEPSILON} controls the Cartesian direction along which | Finally, the tag {{TAG | IEPSILON}} controls the Cartesian direction along which | ||
the delta pulse is applies. | the delta pulse is applies. | ||
Revision as of 12:58, 28 March 2018
Description: 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)
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 [1].
Details of the implementation are explained in Ref. [2]. The time propagation algorithm is VASP uses relatively large timestep, by projecting after each time step onto a specific number of occupied and unoccupied states. The number of occupied and unoccupied pairs are controlled by the tags NBANDSO and NBANDSV and OMEGAMAX - in the same manner as it is 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 calculation.
Per default, the time propagation code includes Hartree and local field effects (LHARTREE=.TRUE. and LFXC=.TRUE.). Results in the independent particle approximation can be calculated by setting LHARTREE=.FALSE. and LFXC=.FALSE. Other combinations (LHARTREE=.TRUE. and LFXC=.FALSE. or LHARTREE=.FALSE. and LFXC=.TRUE. are presently not supported).
The number of timesteps performed in the propagation is usually inverse proportional to the value of CSHIFT. That is a small CSHIFT will require less timestep (but yield a more strongly broadened spectrum). Whereas a small shift CSHIFT will require more timesteps. Typical values of around CSHIFT=0.1 will result in useful spectra. Alternatively, the number of timesteps can be set directly by the tag NELM. In this case, the number of use supplied steps needs to exceed NELM>100 (otherwise, the value in NELM will be disregarded, and the number of timesteps is determined by the tag CSHIFT.
Finally, the tag IEPSILON controls the Cartesian direction along which the delta pulse is applies.
VASP posses multiple other routines to calculate the frequency dependent dielectric function. The simplest approach uses the independent particle approximation (LOPTICS=.TRUE). Furthermore, one can use ALGO = TDHF (BSE calculations equivalent to solving the Casida equation), ALGO = GW (GW calculations). For standard DFT, the timeevolution algorithm is usually fastest, whereas for hybrid functionals ALGO = TDHF is usually faster. Results of timeevolution are strictly identical to ALGO = TDHF and ANTIRES = 2, if CSHIFT, OMEGAMAX NBANDSV, and NBANDSO are chosen identical.
Related Tags and Sections
CSHIFT, LHARTREE, NBANDSV, NBANDSO, OMEGAMAX
References
- ↑ 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.
- ↑ T. Sander, E. Maggio, and G. Kresse, Beyond the Tamm-Dancoff approximation for extended systems using exact diagonalization. Physical Review B, 92, 045209 (2015).