STABLE QUANTUM MONTE-CARLO ALGORITHM FOR T=0 CALCULATION OF IMAGINARYTIME GREEN-FUNCTIONS

We present a numerically stable Quantum Monte Carlo algorithm to calcu late zero-temperature imaginary-time Green functions G(r, tau) for Hub bard type models. We illustrate the efficiency of the algorithm by cal culating the on-site Green function G(r = 0, tau) on 4 x 4 to 12 x 12 lattices for the two-dimensional half-filled repulsive Hubbard model a t U/t = 4. By fitting the tail of G(r = 0, tau) at long imaginary time to the form e(-tau Delta c), we obtain a precise estimate of the char ge gap: Delta(c) 0.67 +/- 0.02 in units of the hopping matrix element. We argue that the algorithm provides a powerful tool to study the met al-insulator transition from the insulator side.