CASINO manual - Theory of Condensed Matter

2 The quantum Monte Carlo method
The correlated motion of electrons plays a crucial role in the aggregation of atoms into molecules
and solids, in electronic transport properties and in many other important physical phenomena. Ab
initio electronic structure calculations, in which the properties of such correlated electron systems are
computed from first principles, are a vital tool in modern condensed matter physics and molecular
quantum chemistry. Currently the most popular way to include the effects of electron correlation in
these calculations is density functional theory (DFT). This method is in principle exact, in reality fast
and often very accurate, but does have a certain number of well-known limitations. In particular, with
only limited knowledge available concerning the exact mathematical form of the so-called exchange–
correlation (XC) functional, the accuracy of the approximate form of the theory is nonuniform and
nonuniversal, and there are important classes of materials for which it gives qualitatively wrong
answers.
An important and complementary alternative for situations where accuracy is paramount is the quantum
Monte Carlo (QMC) method, which has many attractive features for probing the electronic
structure of real systems. It is an explicitly many-body method, applicable to both finite and periodic
systems, which takes electron correlation into account from the outset. It gives consistent, highly accurate
results while at the same time exhibiting favourable scaling of computational cost with system
size. This is in sharp contrast to the accurate methods used in mainstream quantum chemistry, such
as configuration interaction or coupled-cluster theory, which are impractical for anything other than
small molecules, and cannot generally be applied to condensed matter problems.
The use of QMC was greatly hampered over its first two decades of existence by a combination
of insufficient computer power and a lack of available, efficient QMC computer programs general
enough to treat a similar range of problems to regular DFT codes. Fast parallel computers are now
widespread, and you are now in possession of just such a computer program, which is capable of
carrying out QMC calculations for a wide range of interesting chemical and physical problems on a
variety of computational hardware. As a bonus, casino is capable of exploiting with essentially perfect
parallel efficiency as many processors as you like—a great asset in this era of petascale computing
where machines with hundreds of thousands or even millions of cores are beginning to appear. casino
has been designed to make the power of the QMC method available to everyone and we hope you
enjoy using it.
3 Miscellaneous issues
3.1 Support
casino has documentation and examples, etc., but it is a research code, and learning how to use it
is a significant task. This is particularly the case if the user does not have relevant experience, such
as familiarity with the theory of variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC)
calculations and knowledge of DFT or Hartree–Fock (HF) methods for atoms, molecules and solids.
We have found that supporting users can take up a large amount of our time and so we have to
limit the number of groups that we work with directly. Most users need quite a lot of help and their
projects turn into collaborations, but of course we cannot enter into too many projects of this type
as our time is limited. We often have people visiting Cambridge to learn about the codes and how to
do calculations, and this seems to work well.
Beginning in 2006 we also organize annual summer schools at MDT’s Tuscan monastery (see vallico.
net/tti/tti.html). The scientific side of these week-long events involves over 20 hours of lectures
from the main authors of the code with plenty of hands-on tuition, and people who have attended
these events have generally found them to be beneficial.
Since June 2013, the best place for having your questions about casino answered is over at the
casino discussion forum vallico.net/casino-forum. Unless you have a good reason for thinking
some particular individual can help you, this forum should be the first place you ask for help. This
supersedes the earlier mailing list casino-users ‘at’ tcm.phy.cam.ac.uk which still exists but is
now only used by the casino administrators for announcements of, e.g., events and new versions of
the code.
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