  Quantum Monte Carlo (QMC) simulations constitute nowadays one of the most
powerful methods to study strongly correlated quantum systems, provided that no
"sign problem" arises. However, many systems of interest, including highly
frustrated magnets, suffer from an average sign that is close to zero in
standard QMC simulations. Nevertheless, a possible sign problem depends on the
simulation basis, and here we demonstrate how a suitable choice of cluster
basis can be used to eliminate or at least reduce the sign problem in highly
frustrated magnets that were so far inaccessible to efficient QMC simulations.
We focus in particular on the application of a two-spin (dimer)-based QMC
method to the thermodynamics of the spin-1/2 Shastry-Sutherland model for
SrCu$_2$(BO$_3$)$_2$.
