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Title: H{sub 4}: A challenging system for natural orbital functional approximations

The correct description of nondynamic correlation by electronic structure methods not belonging to the multireference family is a challenging issue. The transition of D{sub 2h} to D{sub 4h} symmetry in H{sub 4} molecule is among the most simple archetypal examples to illustrate the consequences of missing nondynamic correlation effects. The resurgence of interest in density matrix functional methods has brought several new methods including the family of Piris Natural Orbital Functionals (PNOF). In this work, we compare PNOF5 and PNOF6, which include nondynamic electron correlation effects to some extent, with other standard ab initio methods in the H{sub 4} D{sub 4h}/D{sub 2h} potential energy surface (PES). Thus far, the wrongful behavior of single-reference methods at the D{sub 2h}–D{sub 4h} transition of H{sub 4} has been attributed to wrong account of nondynamic correlation effects, whereas in geminal-based approaches, it has been assigned to a wrong coupling of spins and the localized nature of the orbitals. We will show that actually interpair nondynamic correlation is the key to a cusp-free qualitatively correct description of H{sub 4} PES. By introducing interpair nondynamic correlation, PNOF6 is shown to avoid cusps and provide the correct smooth PES features at distances close to the equilibrium, totalmore » and local spin properties along with the correct electron delocalization, as reflected by natural orbitals and multicenter delocalization indices.« less
Authors:
;  [1] ; ;  [1] ;  [2]
  1. Faculty of Chemistry, University of the Basque Country UPV/EHU, and Donostia International Physics Center (DIPC), P.K. 1072, 20080 Donostia, Euskadi (Spain)
  2. (Spain)
Publication Date:
OSTI Identifier:
22493148
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 143; Journal Issue: 16; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; 97 MATHEMATICAL METHODS AND COMPUTING; APPROXIMATIONS; COMPARATIVE EVALUATIONS; COUPLING; DENSITY MATRIX; ELECTRON CORRELATION; ELECTRONIC STRUCTURE; ELECTRONS; EQUILIBRIUM; FUNCTIONALS; MOLECULES; POTENTIAL ENERGY; SPIN; SURFACES; SYMMETRY