On the Multi-Reference Nature of Plutonium Oxides: PuO$_2^{2+}$, PuO$_2$, PuO$_3$ and PuO$_2$(OH)$_2$

TL;DR

This paper develops a protocol using quantum information theory and the Density Matrix Renormalization Group to define optimal active spaces for complex plutonium oxides, revealing their multi-reference electronic structure and bonding mechanisms.

Contribution

It introduces a new protocol for active space selection in multi-reference calculations of actinide complexes, enhancing accuracy and understanding of their electronic structure.

Findings

Optimal active spaces accurately describe static electron correlation.

Addition of oxo- or hydroxo-groups alters pi-bonding and structure.

Plutonium oxides exhibit significant multi-reference character.

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 (PuO$_3$ and PuO$_2$(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 pi-bonding mechanism with respect to the bare triatomics, resulting in bent structures with considerable multi-reference character.