MONTE-CARLO EVALUATION OF PATH-INTEGRALS FOR THE NUCLEAR SHELL-MODEL

We present in detail a formulation of the shell model as a path integr al and Monte Carlo techniques for its evaluation. The formulation, whi ch linearizes the two-body interaction by an auxiliary field, is quite general, both in the form of the effective ''one-body'' Hamiltonian a nd in the choice of ensemble. In particular, we derive formulas for th e use of general (beyond monopole) pairing operators, as well as a nov el extraction of the canonical (fixed-particle-number) ensemble via an activity expansion. We discuss the advantages and disadvantages of th e various formulations and ensembles and,give several illustrative exa mples. We also discuss and illustrate calculation of the imaginary-tim e response function and the extraction, by maximum entropy methods, of the corresponding strength function. Finally, we discuss the ''sign p roblem'' generic to fermion Monte Carlo calculations, and prove that a wide class of interactions are free of this limitation.