  Macroscopic assemblies of interacting spins give rise to a broad spectrum of
behaviors determined by the spatial arrangement of the magnetic sites and the
electronic interactions between them. Compounds of copper (II), in which each
copper carries spin $\frac{1}{2}$, exhibit a vast variety of physical
properties. For antiferromagnetically coupled spin sites, there are two
limiting scenarios: spin chains in which the spins can exhibit a long-range
order or a mixture of dimers in which the spins within each pair are entangled
but do not communicate with the spins from other dimers. In principle, the two
types can be distinguished on the basis of experimental observations and
modeling using empirically parameterized effective Hamiltonians, but in
practice, ambiguity may persist for decades, as is the case for copper oxalate.
Here we use high-level ab initio calculations to establish the validity of the
nearest-site Heisenberg model and to predict the interaction strength between
the magnetic sites. The computed magnetic susceptibility provides an
unambiguous interpretation of magnetic experiments performed throughout half a
century, clearly supporting the infinite spin-chain behavior of solid copper
oxalate.
