LFXC - Revision history

Tal: /* Casida equation */

2026-06-15T14:56:28Z

Casida equation

← Older revision		Revision as of 14:56, 15 June 2026
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	The Casida approach requires the derivative to be evaluated explicitly and therefore implemented for each functional. The time-evolution TDDFT does not require an explicit kernel: its contribution is included implicitly through the propagation of the charge density and the exchange-correlation potential.		The Casida approach requires the derivative to be evaluated explicitly and therefore implemented for each functional. The time-evolution TDDFT does not require an explicit kernel: its contribution is included implicitly through the propagation of the charge density and the exchange-correlation potential.

	=== Casida ~~equation~~ ===		=== Casida TDDFT ===

	For an [[:Category:Exchange-correlation functionals\|LDA]] functional,		For an [[:Category:Exchange-correlation functionals\|LDA]] functional,

Tal: /* Compare Casida and time-evolution TDDFT results */

2026-06-05T16:23:19Z

Compare Casida and time-evolution TDDFT results

← Older revision		Revision as of 16:23, 5 June 2026
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	== Compare Casida and time-evolution TDDFT results ==		== Compare Casida and time-evolution TDDFT results ==
	The Casida and time-evolution approaches produce very similar results for LDA exchange-correlation. Small differences typically remain because one-center terms in the PAW method are treated differently in the two approaches. To bring the Casida results into closer agreement, increase {{TAG\|ENCUTGW}} beyond its default value.		The Casida and time-evolution approaches produce very similar results for LDA exchange-correlation. Small differences typically remain because one-center terms in the PAW method are treated differently in the two approaches. To bring the Casida results into closer agreement, increase {{TAG\|ENCUTGW}} beyond its default value and set {{TAG\|ANTIRES}}=2 in the Casida TDDFT calculation.

	== Related tags and articles ==		== Related tags and articles ==

Tal at 15:12, 5 June 2026

2026-06-05T15:12:06Z

@@ Line 1: / Line 1: @@
-{{TAGDEF|LFXC|.TRUE. {{!}} .FALSE.}}
+{{TAGDEF|LFXC|.TRUE. {{!}} .FALSE.|.FALSE.}}
-Description: {{TAG|LFXC}} enables the local exchange-correlation kernel in [[Time-evolution algorithm|TD-DFT]] and [[BSE calculations|TDHF]] calculations.
+Description: {{TAG|LFXC}} enables the (semi-)local exchange-correlation kernel in [[Time-dependent density-functional theory calculations|Casida]] and [[Time-evolution algorithm|time-evolution]] TDDFT calculations.
 ----
-The local exchange-correlation kernel is determined via {{cite|sander:prb:15}}
+In linear-response TDDFT, the density-density response function <math>\chi</math> obeys the Dyson equation
 ::<math>
-f^{\rm loc}_{\mathrm{xc}}\left(\mathbf{r}, \mathbf{r}'\right) =
+f_\mathrm{xc}^\mathrm{GGA}(\mathbf r, \mathbf r') = \frac{\partial^2 \varepsilon_\mathrm{xc}}{\partial n^2}(\mathbf r) \, \delta(\mathbf r - \mathbf r') - \left[\nabla \frac{\partial^2 \varepsilon_\mathrm{xc}}{\partial n \, \partial \nabla n}(\mathbf r)\right] \delta(\mathbf r - \mathbf r') - \nabla_i \frac{\partial^2 \varepsilon_\mathrm{xc}}{\partial \nabla_i n \, \partial \nabla_j n}(\mathbf r) \, \nabla_j \delta(\mathbf r - \mathbf r'),
-\frac{\delta^{2}\left\{E_{\mathrm{c}}^{\mathrm{DFT}}+\left(1-c_{\rm x}\right)
+</math>
-E_{\mathrm{x}}^{\mathrm{DFT}}\right\}}{\delta \rho(\mathbf{r}) \delta
+where <math>i, j</math> are summed Cartesian indices. In the Casida approach these gradient terms are dropped and only the density derivatives are kept. [[:Category:Exchange-correlation functionals|Meta-GGA]] kernels are not supported.
-\rho\left(\mathbf{r}'\right)}
-</math>,
+=== Time-evolution TDDFT (Real-time TDDFT) ===
-[[BSE calculations]], [[Time-evolution_algorithm]]
+The real-time propagation applies <math>f_\mathrm{xc}</math> directly to the time-dependent density, so [[:Category:Exchange-correlation functionals|LDA]] and [[:Category:Exchange-correlation functionals|GGA]] kernels are used in full, including the gradient terms.
 {{sc|LFXC|Howto|Workflows that use this tag}}
 == References ==
 <references/>
-[[Category:INCAR tag]][[Category:Many-body perturbation theory]][[Category:Bethe-Salpeter equations]]
+[[Category:INCAR tag]]

Pmelo at 14:37, 3 November 2025

2025-11-03T14:37:51Z

← Older revision		Revision as of 14:37, 3 November 2025
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	{{DEF\|LFXC\|.FALSE.\|}}		{{DEF\|LFXC\|.FALSE.\|}}

	Description: {{TAG\|LFXC}} enables the local exchange-correlation kernel in [[~~BSE calculations~~\|TD-DFT]] and [[BSE calculations\|TDHF]] calculations.		Description: {{TAG\|LFXC}} enables the local exchange-correlation kernel in [[Time-evolution algorithm\|TD-DFT]] and [[BSE calculations\|TDHF]] calculations.
	----		----
	The local exchange-correlation kernel is determined via {{cite\|sander:prb:15}}		The local exchange-correlation kernel is determined via {{cite\|sander:prb:15}}
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	where <math>c_{\rm x}</math> is the fraction of the exchange interaction set by {{TAG\|AEXX}}.		where <math>c_{\rm x}</math> is the fraction of the exchange interaction set by {{TAG\|AEXX}}.
	== Related tags and articles==		== Related tags and articles==
	[[BSE calculations]]		{{TAG\|LADDER}}, {{TAG\|LHARTREE}}, {{TAG\|AEXX}}

			[[BSE calculations]], [[Time-evolution_algorithm]]

			{{sc\|LFXC\|Howto\|Workflows that use this tag}}


	== References ==		== References ==

Ftran at 08:30, 20 October 2023

2023-10-20T08:30:11Z

← Older revision		Revision as of 08:30, 20 October 2023
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	<references/>		<references/>
	----		----
	~~[[The_VASP_Manual\|Contents]]~~

	[[Category:INCAR tag]][[Category:Many-body perturbation theory]][[Category:Bethe-Salpeter equations]]		[[Category:INCAR tag]][[Category:Many-body perturbation theory]][[Category:Bethe-Salpeter equations]]

Tal: /* Related tags and articles */

2023-09-28T08:49:36Z

Huebsch at 07:36, 26 September 2023

2023-09-26T07:36:12Z

← Older revision		Revision as of 07:36, 26 September 2023
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	</math>,		</math>,
	where <math>c_{\rm x}</math> is the fraction of the exchange interaction set by {{TAG\|AEXX}}.		where <math>c_{\rm x}</math> is the fraction of the exchange interaction set by {{TAG\|AEXX}}.


	== Related tags and articles==		== Related tags and articles==
	{{TAG\|LHARTREE}},		{{TAG\|LHARTREE}},

Tal at 15:01, 20 April 2022

2022-04-20T15:01:40Z

← Older revision		Revision as of 15:01, 20 April 2022
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	where <math>c_{\rm x}</math> is the fraction of the exchange interaction set by {{TAG\|AEXX}}.		where <math>c_{\rm x}</math> is the fraction of the exchange interaction set by {{TAG\|AEXX}}.

	~~{{NB\|warning\| Currently, the local <math>f_{\rm xc}</math> is only evaluated in at the LDA level.\|}}~~

	== Related tags and articles==		== Related tags and articles==

Tal at 08:30, 11 April 2022

2022-04-11T08:30:00Z

← Older revision		Revision as of 08:30, 11 April 2022
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	[[The_VASP_Manual\|Contents]]		[[The_VASP_Manual\|Contents]]

	[[Category:INCAR tag]][[Category:Many-body perturbation		[[Category:INCAR tag]][[Category:Many-body perturbation theory]][[Category:Bethe-Salpeter equations]]
	theory]][[Category:Bethe-Salpeter equations]]')

Tal at 12:05, 8 April 2022

2022-04-08T12:05:31Z

← Older revision		Revision as of 12:05, 8 April 2022
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	where <math>c_{\rm x}</math> is the fraction of the exchange interaction set by {{TAG\|AEXX}}.		where <math>c_{\rm x}</math> is the fraction of the exchange interaction set by {{TAG\|AEXX}}.

	{{NB\|warning\| Currently, the local <math>f_{\rm xc}</math> is only evaluated in the LDA ~~approximation~~.\|}}		{{NB\|warning\| Currently, the local <math>f_{\rm xc}</math> is only evaluated in at the LDA level.\|}}

	== Related tags and articles==		== Related tags and articles==

← Older revision		Revision as of 08:49, 28 September 2023
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	where <math>c_{\rm x}</math> is the fraction of the exchange interaction set by {{TAG\|AEXX}}.		where <math>c_{\rm x}</math> is the fraction of the exchange interaction set by {{TAG\|AEXX}}.
	== Related tags and articles==		== Related tags and articles==
	~~{{TAG\|LHARTREE}},~~
	~~{{TAG\|ISPIN}},~~
	~~{{TAG\|LSORBIT}},~~
	[[BSE calculations]]		[[BSE calculations]]