Hamiltonian eigenvalues are calculated for the (2+1)D Ising model on lattices up to 5*5 sites. Finite-size scaling techniques are used to estimate the critical coupling x c =0.329+or-0.001, and index v=0.635+or-0.005. The prospects are discussed for further applications of these methods in three and four dimensions.
We have performed extensive studies of finite-size scaling in the three-dimensional Ising model, using a specially designed Ising processor. We show results for the susceptibility, the energy and specific heat, and the connected part of the fourth moment of the magnetization. Corrections to finite-size scaling are found to be small only for lattices of linear dimension greater than about L=24. This effect severely limits the accuracy to which critical parameters can be estimated. We estimate Kc=0.221 650\ifmmode\pm\else\textpm\fi{}0.000 005 for the critical coupling, while exponent estimates are consistent with those obtained by other methods.
The method of 'symmetrized' stochastic truncation is applied to the (2+1)-dimensional Ising model, to calculate the ground-state energy, mass gap, magnetization and susceptibility on lattices of sizes up to 6*6 sites. The method is shown to be a powerful and accurate Monte Carlo technique, provided the lattice size is not too large, and the model is suitably 'convergent'. Finite-size scaling estimates of the critical indices are nu =0.627(2), alpha =0.12(2), beta =0.324(3), and delta =1.23(1). An estimate of the combination gamma / nu +2 beta / nu =3.000(1) is obtained, in outstanding agreement with the expected hyperscaling relation.