Category:XAS - Revision history

Csheldon: /* Tutorials */

2026-03-10T13:46:13Z

Tutorials

← Older revision		Revision as of 13:46, 10 March 2026
Line 38:		Line 38:

	== Tutorials ==		== Tutorials ==
	~~Coming soon ...~~
	~~<!--~~
	*Tutorial for [https://vasp.at/tutorials/latest/xas/part1 SCH XAS calculations].		*Tutorial for [https://vasp.at/tutorials/latest/xas/part1 SCH XAS calculations].
	*Tutorial for [https://vasp.at/tutorials/latest/xas/part2 BSE XAS calculations].		*Tutorial for [https://vasp.at/tutorials/latest/xas/part2 BSE XAS calculations].
	~~-->~~

	== References ==		== References ==

	[[Category:VASP\|XAS]][[Category:Linear response]][[Category:Dielectric properties]][[Category:VASP]]		[[Category:VASP\|XAS]][[Category:Linear response]][[Category:Dielectric properties]][[Category:VASP]]

Csheldon at 08:12, 12 May 2025

2025-05-12T08:12:41Z

← Older revision		Revision as of 08:12, 12 May 2025
Line 19:		Line 19:
	In the '''initial state''' approximation, the electronic minimization in the presence of the core hole is not carried out, thus the electron-hole interaction is completely neglected.		In the '''initial state''' approximation, the electronic minimization in the presence of the core hole is not carried out, thus the electron-hole interaction is completely neglected.

	* Learn [[Supercell core-hole calculations\|how to ~~perform an~~ supercell core-hole calculation]]		* Learn [[Supercell core-hole calculations\|how to calculate the XAS via a supercell core-hole calculation]]

	=== Bethe-Salpeter equation ===		=== Bethe-Salpeter equation ===
Line 30:		Line 30:

	The BSE calculation based on the [[Practical guide to GW calculations \|<math>GW</math> calculation]] quasiparticles is considered to be the state of the art for XAS and is overall more reliable and accurate than the SCH methods.		The BSE calculation based on the [[Practical guide to GW calculations \|<math>GW</math> calculation]] quasiparticles is considered to be the state of the art for XAS and is overall more reliable and accurate than the SCH methods.

			* Learn [[Bethe-Salpeter equation for core excitations\|how to calculate the XAS via BSE.]]

	=== Comparing BSE and SCH ===		=== Comparing BSE and SCH ===
	The scaling of the BSE+GW approach is <math>N^4-N^5</math> with the system size, which makes it very expensive computationally for large cells. The SCH approach on the other hand scales as <math>N^3</math> with the system size which makes it much cheaper for complex systems with large cells. Nevertheless, the BSE+GW approach can be less computationally expensive for small cells and provides a better insight into the physics of the excitons. Furthermore, some results indicate that the valence state excitations have to be included in the XAS calculations as well to reproduce the experimental results, which is only possible within the BSE formalism {{cite\|tang:pnas:2022}}. SCH has been shown to provide overall an accurate approximation for the deep core excitations {{cite\|karsai:prb:2018}}, but it can be less accurate for some systems, where the BSE+GW calculations are required to accurately reproduce the experimental spectra {{cite\|unzog:prb:2022}}{{cite\|liang:prl:2017}}.		The scaling of the BSE+GW approach is <math>N^4-N^5</math> with the system size, which makes it very expensive computationally for large cells. The SCH approach on the other hand scales as <math>N^3</math> with the system size which makes it much cheaper for complex systems with large cells. Nevertheless, the BSE+GW approach can be less computationally expensive for small cells and provides a better insight into the physics of the excitons. Furthermore, some results indicate that the valence state excitations have to be included in the XAS calculations as well to reproduce the experimental results, which is only possible within the BSE formalism {{cite\|tang:pnas:2022}}. SCH has been shown to provide overall an accurate approximation for the deep core excitations {{cite\|karsai:prb:2018}}, but it can be less accurate for some systems, where the BSE+GW calculations are required to accurately reproduce the experimental spectra {{cite\|unzog:prb:2022}}{{cite\|liang:prl:2017}}.
	{{NB\|mind\|By default the Lorentzian broadening is applied in the XAS BSE dielectric function. In SCH the default broadening function is Gaussian.}}		{{NB\|mind\|By default the Lorentzian broadening is applied in the XAS BSE dielectric function. In SCH the default broadening function is Gaussian.}}

	~~== How to ==~~
	~~Practical guides on how to perform XAS calculation in different approximations~~

	*XAS via supercell core-hole method: [[Supercell core-hole calculations\|XAS SCH calculations]]
	*XAS via Bethe-Salpeter equation: [[Bethe-Salpeter equation for core excitations\|XAS BSE calculations]]

	== Tutorials ==		== Tutorials ==
Line 49:		Line 45:

	== References ==		== References ==
	~~<references/>~~

	~~<noinclude>~~
	~~----~~

	[[Category:VASP\|XAS]][[Category:Linear response]][[Category:Dielectric properties]][[Category:VASP]]		[[Category:VASP\|XAS]][[Category:Linear response]][[Category:Dielectric properties]][[Category:VASP]]

Csheldon: /* Supercell core-hole (SCH) */

2025-05-12T08:10:32Z

Supercell core-hole (SCH)

← Older revision		Revision as of 08:10, 12 May 2025
Line 18:		Line 18:

	In the '''initial state''' approximation, the electronic minimization in the presence of the core hole is not carried out, thus the electron-hole interaction is completely neglected.		In the '''initial state''' approximation, the electronic minimization in the presence of the core hole is not carried out, thus the electron-hole interaction is completely neglected.

			* Learn [[Supercell core-hole calculations\|how to perform an supercell core-hole calculation]]

	=== Bethe-Salpeter equation ===		=== Bethe-Salpeter equation ===

Csheldon: /* Comparing BSE and SCH */

2025-05-12T07:43:40Z

Comparing BSE and SCH

← Older revision		Revision as of 07:43, 12 May 2025
Line 30:		Line 30:

	=== Comparing BSE and SCH ===		=== Comparing BSE and SCH ===
	The scaling of the BSE+GW approach is <math>N^4-N^5</math> with the system size, which makes it very expensive computationally for large cells. The SCH approach on the other hand scales as <math>N^3</math> with the system size which makes it much cheaper for complex systems with large cells. Nevertheless, the BSE+GW approach can be less computationally expensive for small cells and provides a better insight into the physics of the excitons. Furthermore, some results indicate that the valence state excitations have to be included in the XAS calculations as well to reproduce the experimental results, which is only possible within the BSE formalism {{cite\|tang:pnas:2022\|}}. SCH has been shown to provide overall an accurate approximation for the deep core excitations {{cite\|karsai:prb:2018}}, but it can be less accurate for some systems, where the BSE+GW calculations are required to accurately reproduce the experimental spectra {{cite\|unzog:prb:2022}}{{cite\|liang:prl:2017}}.		The scaling of the BSE+GW approach is <math>N^4-N^5</math> with the system size, which makes it very expensive computationally for large cells. The SCH approach on the other hand scales as <math>N^3</math> with the system size which makes it much cheaper for complex systems with large cells. Nevertheless, the BSE+GW approach can be less computationally expensive for small cells and provides a better insight into the physics of the excitons. Furthermore, some results indicate that the valence state excitations have to be included in the XAS calculations as well to reproduce the experimental results, which is only possible within the BSE formalism {{cite\|tang:pnas:2022}}. SCH has been shown to provide overall an accurate approximation for the deep core excitations {{cite\|karsai:prb:2018}}, but it can be less accurate for some systems, where the BSE+GW calculations are required to accurately reproduce the experimental spectra {{cite\|unzog:prb:2022}}{{cite\|liang:prl:2017}}.
	{{NB\|mind\|By default the Lorentzian broadening is applied in the XAS BSE dielectric function. In SCH the default broadening function is Gaussian.}}		{{NB\|mind\|By default the Lorentzian broadening is applied in the XAS BSE dielectric function. In SCH the default broadening function is Gaussian.}}

Tal: /* Bethe-Salpeter equation */

2025-02-21T09:49:31Z

Bethe-Salpeter equation

← Older revision		Revision as of 09:49, 21 February 2025
Line 20:		Line 20:

	=== Bethe-Salpeter equation ===		=== Bethe-Salpeter equation ===
	Within the BSE approach{{cite\|unzog:prb:2022}}, the interaction between the core hole and the excited electron is explicitly included ~~via~~ the ~~ladder diagrams~~ in the ~~polarizability~~. The BSE is solved in the transition space as an eigenvalue problem, whose eigenvalues <math>\varepsilon^\lambda</math> are the transitions including the excitonic effects and the eigenvectors <math>A^\lambda</math> are the excitonic states in the transition space.		Within the BSE approach{{cite\|unzog:prb:2022}}, the interaction between the core hole and the excited electron is explicitly included in the polarizability as explained in the [[Bethe-Salpeter equation\|theory page]]. The BSE is solved in the transition space as an eigenvalue problem, whose eigenvalues <math>\varepsilon^\lambda</math> are the transitions including the excitonic effects and the eigenvectors <math>A^\lambda</math> are the excitonic states in the transition space.
	The BSE dielectric function is found via		The BSE dielectric function is found via

Karsai: /* Comparing BSE and SCH */

2025-02-20T15:36:45Z

Comparing BSE and SCH

← Older revision		Revision as of 15:36, 20 February 2025
Line 31:		Line 31:
	=== Comparing BSE and SCH ===		=== Comparing BSE and SCH ===
	The scaling of the BSE+GW approach is <math>N^4-N^5</math> with the system size, which makes it very expensive computationally for large cells. The SCH approach on the other hand scales as <math>N^3</math> with the system size which makes it much cheaper for complex systems with large cells. Nevertheless, the BSE+GW approach can be less computationally expensive for small cells and provides a better insight into the physics of the excitons. Furthermore, some results indicate that the valence state excitations have to be included in the XAS calculations as well to reproduce the experimental results, which is only possible within the BSE formalism {{cite\|tang:pnas:2022\|}}. SCH has been shown to provide overall an accurate approximation for the deep core excitations {{cite\|karsai:prb:2018}}, but it can be less accurate for some systems, where the BSE+GW calculations are required to accurately reproduce the experimental spectra {{cite\|unzog:prb:2022}}{{cite\|liang:prl:2017}}.		The scaling of the BSE+GW approach is <math>N^4-N^5</math> with the system size, which makes it very expensive computationally for large cells. The SCH approach on the other hand scales as <math>N^3</math> with the system size which makes it much cheaper for complex systems with large cells. Nevertheless, the BSE+GW approach can be less computationally expensive for small cells and provides a better insight into the physics of the excitons. Furthermore, some results indicate that the valence state excitations have to be included in the XAS calculations as well to reproduce the experimental results, which is only possible within the BSE formalism {{cite\|tang:pnas:2022\|}}. SCH has been shown to provide overall an accurate approximation for the deep core excitations {{cite\|karsai:prb:2018}}, but it can be less accurate for some systems, where the BSE+GW calculations are required to accurately reproduce the experimental spectra {{cite\|unzog:prb:2022}}{{cite\|liang:prl:2017}}.
	{{NB\|mind\|By default the Lorentzian broadening is applied in the XAS BSE dielectric ~~functionis Lorentzian~~. In SCH the default broadening function is Gaussian.}}		{{NB\|mind\|By default the Lorentzian broadening is applied in the XAS BSE dielectric function. In SCH the default broadening function is Gaussian.}}

	== How to ==		== How to ==

Karsai: /* Bethe-Salpeter equation */

2025-02-20T15:21:24Z

Bethe-Salpeter equation

← Older revision		Revision as of 15:21, 20 February 2025
Line 20:		Line 20:

	=== Bethe-Salpeter equation ===		=== Bethe-Salpeter equation ===
	Within the BSE approach{{cite\|unzog:prb:2022}}, the interaction between the core hole and the excited electron is explicitly included via the ladder diagrams in the polarizability.The BSE is solved in the transition space as an eigenvalue problem, whose eigenvalues <math>\varepsilon^\lambda</math> are the transitions including the excitonic effects and the eigenvectors <math>A^\lambda</math> are the excitonic states in the transition space.		Within the BSE approach{{cite\|unzog:prb:2022}}, the interaction between the core hole and the excited electron is explicitly included via the ladder diagrams in the polarizability. The BSE is solved in the transition space as an eigenvalue problem, whose eigenvalues <math>\varepsilon^\lambda</math> are the transitions including the excitonic effects and the eigenvectors <math>A^\lambda</math> are the excitonic states in the transition space.
	The BSE dielectric function is found via		The BSE dielectric function is found via

Karsai: /* How to */

2025-02-20T14:53:36Z

How to

← Older revision		Revision as of 14:53, 20 February 2025
Line 36:		Line 36:
	Practical guides on how to perform XAS calculation in different approximations		Practical guides on how to perform XAS calculation in different approximations

	*XAS via supercell core-hole method: [[Supercell core-hole calculations\|XAS ~~supercell core-hole~~ calculations]]		*XAS via supercell core-hole method: [[Supercell core-hole calculations\|XAS SCH calculations]]
	*XAS via Bethe-Salpeter equation: [[Bethe-Salpeter equation for core excitations\|XAS BSE calculations]]		*XAS via Bethe-Salpeter equation: [[Bethe-Salpeter equation for core excitations\|XAS BSE calculations]]

Karsai: /* Tutorials */

2025-02-20T14:51:58Z

Tutorials

← Older revision		Revision as of 14:51, 20 February 2025
Line 45:		Line 45:
	*Tutorial for [https://vasp.at/tutorials/latest/xas/part2 BSE XAS calculations].		*Tutorial for [https://vasp.at/tutorials/latest/xas/part2 BSE XAS calculations].
	-->		-->

			== References ==
			<references/>

			<noinclude>
	----		----

	[[Category:VASP\|XAS]][[Category:Linear response]][[Category:Dielectric properties]][[Category:VASP]]		[[Category:VASP\|XAS]][[Category:Linear response]][[Category:Dielectric properties]][[Category:VASP]]

Karsai: /* Tutorials */

2025-02-20T14:50:54Z

Tutorials

← Older revision		Revision as of 14:50, 20 February 2025
Line 41:		Line 41:
	== Tutorials ==		== Tutorials ==
	Coming soon ...		Coming soon ...
	<!-- *Tutorial for [https://vasp.at/tutorials/latest/xas/~~par1~~ SCH XAS calculations].		<!--
	~~<!--~~ *Tutorial for [https://vasp.at/tutorials/latest/xas/~~par2~~ BSE XAS calculations].		*Tutorial for [https://vasp.at/tutorials/latest/xas/part1 SCH XAS calculations].
			*Tutorial for [https://vasp.at/tutorials/latest/xas/part2 BSE XAS calculations].
			-->
	----		----

	[[Category:VASP\|XAS]][[Category:Linear response]][[Category:Dielectric properties]][[Category:VASP]]		[[Category:VASP\|XAS]][[Category:Linear response]][[Category:Dielectric properties]][[Category:VASP]]