My PhD thesis - Condensed Matter Theory - Imperial College London

Chapter 1
Introduction
In recent decades, the use of computer simulation as a tool complementary to traditional
experiment has become widely accepted. In many situations, ‘real-life’
experiments are currently beyond our capabilities; in others, the difficulty of the
experiments means that the results are open to interpretation.
One area in which simulations have become ubiquitous is electronic structure.
Here, the most widely-used computational tool is density-functional theory (DFT);
an indication of the significance and impact of this method is the decision to award
the 1998 Nobel Prize for Chemistry to its creator, Walter Kohn. Electronic structure
is also the category into which this thesis falls, although the focus is not on DFT
but on another technique: quantum Monte Carlo (QMC) simulation.
While DFT has become hugely popular, and has relatively low computational
cost, it is not regarded as the most accurate tool available. For many systems, the
benchmark against which other techniques are judged is QMC, and in particular,
the version of QMC known as diffusion Monte Carlo (DMC). In fact, DMC results
are used to model the function at the heart of DFT calculations: the exchangecorrelation
potential.
QMC calculations are computationally expensive, and despite efforts to devise
faster, more efficient algorithms, will never be able to match DFT in this regard.
Their relevance comes from the need for greater accuracy than can be provided by
other techniques.
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