Title: Exchange constants in molecule-based magnets derived from density functional methods,
Abstract: Cu(pyz)(NO3)2 is a quasi one-dimensional molecular antiferromagnet that
exhibits three dimensional long-range magnetic order below TN=110 mK due to the
presence of weak inter-chain exchange couplings. Here we compare calculations
of the three largest exchange coupling constants in this system using two
techniques based on plane-wave basis-set density functional theory: (i) a dimer
fragment approach and (ii) an approach using periodic boundary conditions. The
calculated values of the large intrachain coupling constant are found to be
consistent with experiment, showing the expected level of variation between
different techniques and implementations. However, the interchain coupling
constants are found to be smaller than the current limits on the resolution of
the calculations. This is due to the computational limitations on convergence
of absolute energy differences with respect to basis set, which are larger than
the inter-chain couplings themselves. Our results imply that errors resulting
from such limitations are inherent in the evaluation of small exchange
constants in systems of this sort, and that many previously reported results
should therefore be treated with caution.