 
Summary: Stochastic Coupled Cluster Theory
Alex J. W. Thom
Department of Chemistry, Imperial College London, London SW7 2AZ, U.K. and
University of Cambridge, Chemistry Department, Lensfield Road, Cambridge CB2 1EW, U.K.
(Dated: December 4, 2010)
We describe a Stochastic Coupled Cluster Theory which represents excitation amplitudes as dis
crete excitors in the space of excitation amplitudes. Reexpressing the Coupled Cluster (CC) equa
tions as the dynamics of `excitors' in this space, we show that a simple set of rules suffice to evolve
a distribution of excitors to sample the CC solution and correctly evaluate the CC energy. These
rules are not truncationspecific and this method can calculate CC solutions to an arbitrary level of
truncation. We present results of calculation on the neon atom, and nitrogen and water molecules
showing the ability to recover both truncated and full CC results.
Coupled Cluster Theory has been said to be the gold
standard of electronic structure methods, and is routinely
used in many branches of physics[13]. With the addition
of perturbatitive corrections, CCSD(T) can routinely
produce energies well within `chemical accuracy'[4], of
1 kcal/mol. It has the desirable property that trunca
tions of the theory are size extensive and rapidly con
verge with increasing truncation level to give spectro
