Stable Quantum Monte Carlo Algorithm forT=0Calculation of Imaginary Time Green Functions

We present a numerically stable Quantum Monte Carlo algorithm to calculate zero-temperature imaginary-time Green functions G(r⃗, τ) for Hubbard type models. We illustrate the efficiency of the algorithm by calculating the on-site Green function G(r⃗=0, τ) on 4 × 4 to 12 × 12 lattices for the two-dimensional half-filled repulsive Hubbard model at $U/t = 4$. By fitting the tail of G(r⃗=0, τ) at long imaginary time to the form e-τ Δc, we obtain a precise estimate of the charge gap: Δc = 0.67 ± 0.02 in units of the hopping matrix element. We argue that the algorithm provides a powerful tool to study the metal-insulator transition from the insulator side.