Importance of anisotropy in the spin-liquid candidate Me3EtSb[Pd(dmit)(2)](2)

Organic charge-transfer salts based on the molecule Pd(dmit)(2) display strong electronic correlations and geometrical frustration, leading to spin-liquid, valence bond solid, and superconducting states, among other interesting phases. The low-energy electronic degrees of freedom of these materials are often described by a single band model: a triangular lattice with a molecular orbital representing a Pd(dmit)(2) dimer on each site. We use ab initio electronic structure calculations to construct and parametrize low-energy effective model Hamiltonians for a class of Me4-n EtnX[Pd(dmit)(2)](2) (X = As, P, N, Sb) salts and investigate how best to model these systems by using variational Monte Carlo simulations. Our findings suggest that the prevailing model of these systems as a t - t' triangular lattice is incomplete and that a fully anisotropic triangular lattice description produces importantly different results, including a significant lowering of the critical U of the spin-liquid phase.