  An ab-initio method for determining the dynamical structure function of an
interacting many--body quantum system has been devised by combining a
generalized integral transform method with Quantum Monte Carlo methods. As a
first application, the coherent and, separately, the incoherent excitation
spectrum of bulk atomic 4He has been computed, both in the low and intermediate
momentum range. The peculiar form of the kernel in the integral transform of
the dynamical structure function allows to predict, without using any model,
both position and width of the collective excitations in the maxon--roton
region, as well as the second collective peak. A prediction of the dispersion
of the single--particle modes described by the incoherent part is also
presented.
