UNBIASED ESTIMATORS IN QUANTUM MONTE-CARLO METHODS - APPLICATION TO LIQUID-HE-4

A Monte Carlo algorithm for computing quantum-mechanical expectation v alues of coordinate operators in many-body problems is presented. The algorithm, which relies on the forward walking method, fits naturally in a Green's function Monte Carlo calculation, i.e., it does not requi re side walks or a bilinear sampling method. Our method evidences stab ility regions large enough to accurately sample unbiased pure expectat ion values. The proposed algorithm yields accurate results when it is applied to test problems such as the hydrogen atom and the hydrogen mo lecule. An excellent description of several properties of a fully many -body problem such as liquid He-4 at aero temperature is achieved.