Correlated wave functions and the absence of long-range order in numerical studies of the Hubbard model

We present a formulation of the constrained path Monte Carlo method for fermions that uses trial wave functions that include many-body effects. This formulation allows us to implement a whole family of generalized mean-field states as constraints. As an example, we calculated superconducting pairing correlation functions for the two-dimensional repulsive Hubbard model using a BCS trial state as the constraint. We compared the results with the case where a free-electron trial wave function is used. We found that the correlation functions are {ital independent} of which state is used as the constraint, which reaffirms the results previously found by Zhang {ital et al.} [Phys. Rev. Lett. {bold 78}, 4486 (1997)] regarding the suppression of long-range pairing correlations as the system size increases. {copyright} {ital 1999} {ital The American Physical Society}