title: On the multi-reference nature of plutonium oxides: PuO22+, PuO2, PuO3and PuO2(OH)2.
abstract: Actinide-containing complexes present formidable challenges for electronic structure methods due to the large number of degenerate or quasi-degenerate electronic states arising from partially occupied 5f and 6d shells. Conventional multi-reference methods can treat active spaces that are often at the upper limit of what is required for a proper treatment of species with complex electronic structures, leaving no room for verifying their suitability. In this work we address the issue of properly defining the active spaces in such calculations, and introduce a protocol to determine optimal active spaces based on the use of the Density Matrix Renormalization Group algorithm and concepts of quantum information theory. We apply the protocol to elucidate the electronic structure and bonding mechanism of volatile plutonium oxides (PuO3and PuO2(OH)2), species associated with nuclear safety issues for which little is known about the electronic structure and energetics. We show how, within a scalar relativistic framework, orbital-pair correlations can be used to guide the definition of optimal active spaces which provide an accurate description of static/non-dynamic electron correlation, as well as to analyse the chemical bonding beyond a simple orbital model. From this bonding analysis we are able to show that the addition of oxo- or hydroxo-groups to the plutonium dioxide species considerably changes the π-bonding mechanism with respect to the bare triatomics, resulting in bent structures with a considerable multi-reference character.
authors.affiliation: Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University in Torun, Grudziadzka 5, 87-100 Torun, Poland. k.boguslawski@fizyka.umk.pl ptecmer@fizyka.umk.pl and Faculty of Chemistry, Nicolaus Copernicus University in Torun, Gagarina 7, 87-100 Torun, Poland.
authors.forename: Katharina
authors.identifier: http://orcid.org/0000-0001-7793-1151
authors.initials: K
authors.lastname: Boguslawski
authors.affiliation: Univ. Lille, CNRS, UMR 8523 - PhLAM - Physique des Lasers Atomes et Molécules, F-59000 Lille, France. valerie.vallet@univ-lille1.fr.
authors.forename: Florent
authors.initials: F
authors.lastname: Réal
authors.affiliation: Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University in Torun, Grudziadzka 5, 87-100 Torun, Poland. k.boguslawski@fizyka.umk.pl ptecmer@fizyka.umk.pl.
authors.forename: Paweł
authors.initials: P
authors.lastname: Tecmer
authors.affiliation: Univ. Lille, CNRS, UMR 8523 - PhLAM - Physique des Lasers Atomes et Molécules, F-59000 Lille, France. valerie.vallet@univ-lille1.fr and Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, L8S 4M1, Canada.
authors.forename: Corinne
authors.initials: C
authors.lastname: Duperrouzel
authors.affiliation: Univ. Lille, CNRS, UMR 8523 - PhLAM - Physique des Lasers Atomes et Molécules, F-59000 Lille, France. valerie.vallet@univ-lille1.fr.
authors.forename: André Severo Pereira
authors.identifier: http://orcid.org/0000-0002-5437-2251
authors.initials: AS
authors.lastname: Gomes
authors.affiliation: Strongly Correlated Systems "Lendület" Research Group, Wigner Research Center for Physics, H-1525 Budapest, Hungary.
authors.forename: Örs
authors.initials: Ö
authors.lastname: Legeza
authors.affiliation: Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, L8S 4M1, Canada.
authors.forename: Paul W
authors.initials: PW
authors.lastname: Ayers
authors.affiliation: Univ. Lille, CNRS, UMR 8523 - PhLAM - Physique des Lasers Atomes et Molécules, F-59000 Lille, France. valerie.vallet@univ-lille1.fr.
authors.forename: Valérie
authors.identifier: http://orcid.org/0000-0002-2202-3858
authors.initials: V
authors.lastname: Vallet