Simulations of the standard three-dimensional Ising model with a varie
ty of boundary conditions, for medium and large systems, are compared
with field-theoretical predictions. In particular, setting boundary sp
ins equal to zero is not the best approximation for the Dirichlet boun
dary condition of zero magnetization at the boundary. The simulations
for the specific heat agree semiquantitatively with the field theory f
or periodic and Dirichlet boundary conditions.