  We investigate the effect that the temperature dependence of the crystal
structure of a two dimensional organic charge-transfer salt has on the
low-energy Hamiltonian representation of the electronic structure. For that, we
determine the crystal structure of kappa-(BEDT-TTF)2Cu2(CN)3 for a series of
temperatures between T=5 K and 300 K by single crystal X-ray diffraction and
analyze the evolution of the electronic structure with temperature by using
density functional theory and tight binding methods. We find a considerable
temperature dependence of the corresponding triangular lattice Hubbard
Hamiltonian parameters. We conclude that even in the absence of change of
symmetry, the temperature dependence of quantities like frustration and
interaction strength can be significant and should be taken into account.
