Lithuanian Journal of Physics article / Lietuvos fizikos žurnalo straipsnis

SECOND-ORDER RAYLEIGH–SCHRÖDINGER PERTURBATION THEORY FOR THE GRASP2018 PACKAGE: THREE-PARTICLE FEYNMAN DIAGRAM CONTRIBUTION TO VALENCE–VALENCE CORRELATIONS*
Gediminas Gaigalas, Pavel Rynkun, and Laima Kitovienė
Institute of Theoretical Physics and Astronomy, Faculty of Physics, Vilnius University, Saulėtekio 3, 10257 Vilnius, Lithuania
Emails: gediminas.gaigalas@tfai.vu.lt; pavel.rynkun@tfai.vu.lt; laima.radziute@tfai.vu.lt

Received 3 June 2025; revised 16 June 2025; accepted 18 June 2025

The method based on the second-order perturbation theory to identify the most important configuration state functions of various correlations is extended to include valence–valence correlations, which are described by the three-particle Feynman diagram. The extension presented in this work complements the core–valence, core, core–core and valence–valence correlations which were developed in a series of previous papers by G. Gaigalas, P. Rynkun, and L. Kitovienė. Whereas these valence–valence correlations are described by the three-particle Feynman diagram, additional developments to calculate the spin-angular parts of this diagram have been made to the program library librang $\mathtype{librang}$ of the GRASP. As an example of the application of the developed method, the atomic calculations of the energy structure for the Se III ion are presented. In the present work, this method was also used to select the most significant configuration state functions and to use this basis to solve the self-consistent field equations.
Keywords: configuration interaction, spin-angular integration, perturbation theory, tensorial algebra, valence–valence correlations, core–valence correlations, core correlations, core–core correlations
* Dedicated to the memory of professor Zenonas Rokus Rudzikas (1940–2011) on the occasion of his birth anniversary

ANTROSIOS EILĖS RELĖJAUS IR ŠRĖDINGERIO TRIKDYMŲ TEORIJA GRASP2018 PROGRAMINIAM PAKETUI: TRIJŲ DALELIŲ FEINMANO DIAGRAMOS ĮTAKA VALENTINĖMS–VALENTINĖMS KORELIACIJOMS*
Gediminas Gaigalas, Pavel Rynkun, Laima Kitovienė

Vilniaus universiteto Fizikos fakulteto Teorinės fizikos ir astronomijos institutas, Vilnius, Lietuva

Antrosios eilės trikdymų teorija paremtas metodas, skirtas įvairių koreliacijų vertei nustatyti, išplėstas įtraukiant valentines–valentines koreliacijas, kurios aprašomos trijų dalelių Feinmano diagrama. Šis išplėtimas papildo kamieno–valentines, kamieno, kamieno–kamieno ir valentines–valentines koreliacijas, kurios buvo išplėtotos ankstesniuose G. Gaigalo, P. Rynkuno ir L. Kitovienės darbuose. Kadangi šios valentinės–valentinės koreliacijos yra aprašomos trijų dalelių Feinmano diagrama, buvo atlikti papildomi pakeitimai ir išplėtimai Grasp programos bibliotekoje „librang“, leidžiantys apskaičiuoti šios diagramos sukinines-kampines dalis. Darbe, kaip metodo taikymo pavyzdys, taip pat pateikiami Se III energijos struktūros skaičiavimai. Be to, išplėstasis metodas buvo pritaikytas atrinkti svarbiausias konfigūracinių būsenų funkcijas ir panaudoti jas suderintinio lauko lygtims spręsti.

* Skiriama profesoriaus Zenono Rokaus Rudziko (1940–2011) atminimui jo gimimo sukakties proga

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