  We present a numerical study of the evolution of molecular clouds, from their
formation by converging flows in the warm ISM, to their destruction by the
ionizing feedback of the massive stars they form. We improve with respect to
our previous simulations by including a different stellar-particle formation
algorithm, which allows them to have masses corresponding to single stars
rather than to small clusters, and with a mass distribution following a
near-Salpeter stellar IMF. We also employ a simplified radiative-transfer
algorithm that allows the stellar particles to feed back on the medium at a
rate that depends on their mass and the local density. Our results are as
follows: a) Contrary to the results from our previous study, where all stellar
particles injected energy at a rate corresponding to a star of ~ 10 Msun, the
dense gas is now completely evacuated from 10-pc regions around the stars
within 10-20 Myr, suggesting that this feat is accomplished essentially by the
most massive stars. b) At the scale of the whole numerical simulations, the
dense gas mass is reduced by up to an order of magnitude, although star
formation (SF) never shuts off completely, indicating that the feedback
terminates SF locally, but new SF events continue to occur elesewhere in the
clouds. c) The SF efficiency (SFE) is maintained globally at the ~ 10% level,
although locally, the cloud with largest degree of focusing of its accretion
flow reaches SFE ~ 30%. d) The virial parameter of the clouds approaches unity
before the stellar feedback begins to dominate the dynamics, becoming much
larger once feedback dominates, suggesting that clouds become unbound as a
consequence of the stellar feedback. e) The erosion of the filaments that feed
the star-forming clumps produces chains of isolated dense blobs reminiscent of
those observed in the vicinity of the dark globule B68.
