  We introduce a novel parametrization scheme for the Hilbert space of a
spin-1/2 Heisenberg antiferromagnet (AFM) based on an octapartite description
of the square lattice. Our formulation provides an efficient yet systematic way
to model the singlet ground-state wave function within a truncated basis. We
demonstrate its effectiveness by using exact diagonalization to study systems
with up to 64 spins. At significantly reduced computational cost, we obtain
ground-state energies within less than 1% of the lowest values published. The
non-iterative nature of this formulation and the fact that spatial symmetries
remain unexploited lead to opportunities for extensions such as the
incorporation into established iterative methods and the application to
interacting models, for example in the study of the propagation of charged
fermions in a 2D antiferromagnet.
