  We present a general strategy to extend quantum cluster algorithms for S=1/2
spin systems, such as the loop algorithm, to systems with arbitrary size of
spins. In general, the partition function of a high-S spin system is
represented in terms of the path integral of a S=1/2 model with special
boundary conditions. We introduce additional graphs to be assigned to the
boundary part and give the labeling probability explicitly, which completes the
algorithm together with an existing S=1/2 cluster algorithm. As a demonstration
of the algorithm, we simulate the the integer-spin antiferromagnetic Heisenberg
chains. The magnitude of the first excitation gap is estimated as to be
0.41048(6), 0.08917(4), and 0.01002(3) for S=1, 2, and 3, respectively.

Statistical Mechanics