122
A. Łachma´ nska et al.
1 Introduction
One of the main goals of quantum chemistry is to understand the physicochemical
properties of atoms, molecules, and materials using the first principles. This knowledge can be further used to interpret and explain existing experimental data or to
design new compounds with much sought-after properties. However, the molecules
under investigation usually contain numerous interacting electrons, which leads to
a complex computational problem with a large number of degrees of freedom. The
interplay between relativistic effects, the correlated motion of electrons, and the basis
set quality is the main difficulty that limits the possibility to express the electronic
wave function in exact form. Various quantum chemical methods have been successfully applied to molecular systems where these effects play a minor role. However,
molecules containing heavy elements like actinides or other d- and f-block elements
still pose a challenge to quantum chemistry as both correlation and relativistic effects
have a dominant contribution to their electronic structure.
In this chapter, we review conventional and unconventional quantum chemical
theories that are applicable to heavy-element chemistry like actinide-containing compounds. Our discussion starts with presenting the properties of actinides as an example of complex many-electron systems. Then, we briefly summarize some popular
approaches that account for relativistic effects, followed by electronic structure methods that optimize (approximate) electronic wave functions for ground and excited
states. Furthermore, we outline how information from the electronic wave function
can be extracted to obtain a qualitative interpretation of electronic structures. Specifically, our analysis covers concepts of quantum information theory. Finally, we present
some challenging examples of computational actinide chemistry that highlight the
difficulty in describing the electronic structure of actinide-containing compounds.
2 A Brief Overview of Actinides and Their Complex
Electronic Structure
Heavy elements with atomic numbers ranging from 89 to 103 form a distinct group
in the periodic table known as actinides. This series includes actinium, the early
actinides (thorium, protactinium, uranium, and neptunium), the middle actinides
(plutonium, americium, curium, berkelium, and californium), and the late actinides
(einsteinium, fermium, mendelevium, nobelium, and lawrencium). All elements are
radioactive metals and almost all of them are characterized by short lifetimes. Only
some isotopes of thorium and uranium elements have long lifetimes and thus can be
found in nature. Thorium, uranium, neptunium, plutonium, americium, and curium
have important applications in the nuclear industry, whereas thorium and uranium
are also exploited in catalysis.
A. Łachma´ nska et al.
1 Introduction
One of the main goals of quantum chemistry is to understand the physicochemical
properties of atoms, molecules, and materials using the first principles. This knowledge can be further used to interpret and explain existing experimental data or to
design new compounds with much sought-after properties. However, the molecules
under investigation usually contain numerous interacting electrons, which leads to
a complex computational problem with a large number of degrees of freedom. The
interplay between relativistic effects, the correlated motion of electrons, and the basis
set quality is the main difficulty that limits the possibility to express the electronic
wave function in exact form. Various quantum chemical methods have been successfully applied to molecular systems where these effects play a minor role. However,
molecules containing heavy elements like actinides or other d- and f-block elements
still pose a challenge to quantum chemistry as both correlation and relativistic effects
have a dominant contribution to their electronic structure.
In this chapter, we review conventional and unconventional quantum chemical
theories that are applicable to heavy-element chemistry like actinide-containing compounds. Our discussion starts with presenting the properties of actinides as an example of complex many-electron systems. Then, we briefly summarize some popular
approaches that account for relativistic effects, followed by electronic structure methods that optimize (approximate) electronic wave functions for ground and excited
states. Furthermore, we outline how information from the electronic wave function
can be extracted to obtain a qualitative interpretation of electronic structures. Specifically, our analysis covers concepts of quantum information theory. Finally, we present
some challenging examples of computational actinide chemistry that highlight the
difficulty in describing the electronic structure of actinide-containing compounds.
2 A Brief Overview of Actinides and Their Complex
Electronic Structure
Heavy elements with atomic numbers ranging from 89 to 103 form a distinct group
in the periodic table known as actinides. This series includes actinium, the early
actinides (thorium, protactinium, uranium, and neptunium), the middle actinides
(plutonium, americium, curium, berkelium, and californium), and the late actinides
(einsteinium, fermium, mendelevium, nobelium, and lawrencium). All elements are
radioactive metals and almost all of them are characterized by short lifetimes. Only
some isotopes of thorium and uranium elements have long lifetimes and thus can be
found in nature. Thorium, uranium, neptunium, plutonium, americium, and curium
have important applications in the nuclear industry, whereas thorium and uranium
are also exploited in catalysis.