Fixed-node DMC for Fermions on a lattice: Application to the doped Fullerides

Fullerides are crystals made from C60 molecules. When doped with alkali metals the valence electrons of the alkali atoms are transfered to the C60 molecules. We show how to set up a model Hamiltonian to describe the correlated hopping of these electrons between C60 molecules. Then we discuss Monte Carlo methods for such lattice Hamiltonians, putting special emphasis on the optimization of Gutzwiller wavefunctions both in variational and fixed-node diffusion Monte Carlo. As application of these methods we analyze the metal-insulator transition in the integer-doped Fullerides.