The world around us is made of *condensed matter*, i.e. matter whose energy
is low enough that it has condensed to form stable systems of atoms and molecules,
usually in solid or liquid phases. The large variety of ways in which
these systems can take form leads to a rich diversity of physical phenomena
that is practically endless in scope.

Because of this, approaching the field of condensed matter physics from a theoretical or computational
angle can be a very challenging task to undertake. For the most part, the way this
is done is to pick a particular macroscopic phenomenon, which has been well studied
experimentally, and to build empirical, or semi-empirical, models to describe
the experimentally observed results. This often provides a good understanding of
the physics of the system under study, and it is often possible to interpolate or
extrapolate these models in order to predict the behaviour of systems under conditions
not yet tested experimentally. However, due to the complexity of condensed matter systems,
and the difficulty in building accurate models,
the predictive power of such an approach can be severely limited.

The *first principles* approach to condensed matter theory is entirely different from this.
It starts from what we know about all condensed matter systems - that they are made of atoms, which
in turn are made of a positively charged nucleus, and a number of negatively charged electrons. The
interactions between atoms, such as chemical and molecular bonding, are determined by the
interactions of their constituent electrons and nuclei.
All of the physics of condensed matter systems arises
ultimately from these basic interactions. If we can model these interactions accurately,
then all of the complex physical phenomena that arise from them should emerge naturally in our
calculations.

The physics that describes the interaction of electrons and nuclei that is relevant to most
problems in condensed matter is actually relatively simple. There are only two different
types of particle involved, and the behaviour of these particles is mostly governed by basic
quantum mechanics. What makes first principles calculations difficult is
not so much the complexity of the physics, but rather
the size of the problem in terms of a numerical formulation. The development
of accurate and efficient theoretical and computational techniques for dealing with
so many particles is therefore central to the ongoing research in this field.