For a set of N identical massive boson wave packets with optimal initial quantum mechanical localization, we calculate the Hanbury-Brown–Twiss (HBT) two-particle correlation function. Our result provides an algorithm for calculating one-particle spectra and two-particle correlations from an arbitrary phase space occupation (qi,pi,ti)i=1,N as, e.g., returned by event generators. It is a microscopic derivation of the result of the coherent state formalism, providing explicit finite multiplicity corrections. Both the one- and two-particle spectra depend explicitly on the initial wave packet width σ which parametrizes the quantum mechanical wave packet localization. They provide upper and lower bounds which suggest that a realistic value for σ has the order of the Compton wavelength.