$x$-[Pd(dmit)${}_2$]${}_2$ as a quasi-1D, scalene Heisenberg model

From first principles, we calculate the Heisenberg interactions between neighboring dimers in several compounds within the Et${}_n$Me${}_{4-n}X$[Pd(dmit)${}_2$]${}_2$ (Et = ethyl, Me = methyl, dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate) family using an atomistic approach; with broken-symmetry density functional theory. In all materials, we find a scalene triangular model where the strongest exchange coupling along one crystallographic axis is up to three times larger than the others and that frustration further enhances this quasi-one-dimensionality. We calculate the N'eel ordering temperature via the chain random phase approximation (CRPA). We show that the difference in the frustrated interchain couplings is equivalent to a single bipartite interchain coupling, favoring long-range magnetic order. We find that the N'eel ordering temperatures are in good agreement with the experimentally measured values for most compounds.