  Quantum Monte Carlo methods are sophisticated numerical techniques for
simulating interacting quantum systems. In some cases, however, they suffer
from the notorious "sign problem" and become too inefficient to be useful. A
recent publication [J. Wojtkiewicz, Phys. Rev. B 75, 174421 (2007)] claims to
have solved the sign problem for a certain class of frustrated quantum spin
systems through the use of a bipartite valence bond basis. We show in this
Comment that the apparent positivity of the path integral is due to a
misconception about the resolution of the identity operator in this basis, and
that consequently the sign problem remains a severe obstacle for the simulation
of frustrated quantum magnets.
