  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.
