  Problems of temperature behavior of specific heat are solved by the entropy
simulation method for Ising models on a simple square lattice and a square spin
ice (SSI) lattice with nearest neighbor interaction, models of hexagonal
lattices with short-range (SR) dipole interaction, as well as with long-range
(LR) dipole interaction and free boundary conditions, and models of spin
quasilattices with finite interaction radius. It is established that systems of
a finite number of Ising spins with LR dipole interaction can have unusual
thermodynamic properties characterized by several specific-heat peaks in the
absence of an external magnetic field. For a parallel multicanonical sampling
method, optimal schemes are found empirically for partitioning the space of
states into energy bands for Ising and SSI models, methods of concatenation and
renormalization of histograms are discussed, and a flatness criterion of
histograms is proposed. It is established that there is no phase transition in
a model with nearest neighbor interaction on a hexagonal lattice, while the
temperature behavior of specific heat exhibits singularity in the same model,
in case of LR interaction. A spin quasilattice is found that exhibits a nonzero
value of residual entropy.
