Towards a multiconfigurational description of the electronic structure in solids

Materials of ionic crystals are ubiquitous in industrial chemistry.
For example, materials such as cerium dioixde (CeO2) are used in both self-cleaning ovens and to clean exhaust fumes from cars.
Other materials, such as titanium dioixde (TiO2) has been used in the solar-cell industry.
So-called garnets are used in several lasers.

In common for all of these areas of application, is that they are dependant on the motion of the electrons in these materials.
In order to understand how electrons behave and interact, quantum mechanics is required.
A major problem that immediately arises when applying quantum mechanics to crystalline materials, is that crystals are, from a quantum mechanical perspective, enormous.
One single crystal can contain as many as Avogradro's number of atoms.
Quantum mechanical calculation are very demanding, with even the most approximate methods available today being limited to around 10 000 atoms.
The type of methods used in this thesis, generally known as wavefunction theory, are roughly limited to around 100 atoms, depending a bit on what part of the periodic table that is explored and what type of property that is studied.

Methods that fall within wavefunction theory have the advantage against more approximate methods that they follow a fairly strict ladder of increasing accuracy.
In other words, the predicted results can, in principle, be improved by choosing methods from higher up on the ladder.
Of course, the higher up on the ladder a method is, the more computationally expensive it is.
It is therefore not necessarily affordable to move enough steps on the ladder, such that the desired accuracy can be reached.
For that reason, there needs to be some form om compromise when modelling crystals -- in order to improve the description of the electronic structure, the atomic structure has to become more approximate.
Models of that kind are usually referred to as embedding methods.

The purpose of this thesis has been to develop an embedding method for crystalline ionic materials.
This was achieved by developing a computer code called SCEPIC, that generates so-called ab-inito model potentials.
As a part of this thesis work, this method was evaluated in order to provide guidance to other researchers on how to best apply this method.