Spin models in three dimensions: Adaptive lattice spacing

Aiming at the study of critical phenomena in the presence of boundaries with a nontrivial shape we discuss how lattices with an adaptive lattice spacing can be implemented. Since the parameters of the Hamiltonian transform nontrivially under changes of the length scale, adapting the lattice spacing is much more difficult than in the case of the numerical solution of partial differential equations, where this method is common practice. Here we shall focus on the universality class of the three-dimensional Ising model. Our starting point is the improved Blume-Capel model on the simple cubic lattice. In our approach, the system is composed of sectors with lattice spacing a,2a,4a,.... We work out how parts of the lattice with lattice spacing a and 2a, respectively, can be coupled in a consistent way. Here, we restrict ourself to the case where the boundary between the sectors is perpendicular to one of the lattice axes. Based on the theory of defect planes one expects that it is sufficient to tune the coupling between these two regions. To this end we perform a finite-size scaling study. However, first numerical results show that slowly decaying corrections remain. It turns out that these corrections can be removed by adjusting the strength of the couplings within the boundary layers. As a benchmark, we simulate films with strongly symmetry breaking boundary conditions. We determine the magnetization profile and the thermodynamic Casimir force. For our largest thickness L0=64.5, we find that results obtained for the homogeneous system are nicely reproduced.