  We describe a systematic method for complete enumeration of configuration
classes (CCs) of the spin-1/2 Ising model in the energy-magnetization plane.
This technique is applied to the antiferromagnetic Ising model in an external
magnetic field on the square lattice, which is simulated using the tomographic
entropic sampling algorithm. We estimate the number of configurations,
$\Omega(E,m,L)$, and related microcanonical averages, for all allowed energies
$E$ and magnetizations $m$ for $L = 10$ to 30, with $\Delta L = 2$. With prior
knowledge of the CCs, we can be sure that all allowed classes are sampled
during the simulation. Complete enumeration of CCs also enables us to use the
final estimate of $\Omega(E,m,L)$ to obtain good initial estimates,
$\Omega_0(E,m,L')$, for successive system sizes ($L' > L$) through a
two-dimensional interpolation. Using these results we calculate canonical
averages of the thermodynamic quantities of interest as continuous functions of
temperature $T$ and external field $h$. In addition, we determine the critical
line in the $h$-$T$ plane using finite-size scaling analysis, and compare these
results with several approximate theoretical expressions.
