First principles perspective on the microscopic model for Cs2CuCl4 and   Cs2CuBr4
  We investigate the microscopic model for the frustrated layered antiferromagnets Cs2CuCl4 and Cs2CuBr4 by performing ab initio density functional theory (DFT) calculations and with the help of the tight-binding method. The combination of both methods provide the relevant interaction paths in these materials, and we estimate the corresponding exchange constants. We find for Cs2CuCl4 that the calculated ratio of the strongest in-plane exchange constants J'/J between the spin-1/2 Cu ions agrees well with neutron scattering experiments. The microscopic model based on the derived exchange constants is tested by comparing the magnetic susceptibilities obtained from exact diagonalization with experimental data. The electronic structure differences between Cs2CuCl4 and Cs2CuBr4 are also analyzed. 
K. Foyevtsova, Y. Zhang, H. O. Jeschke, R. Valenti