  In spite of decades of theoretical efforts, the physical origin of the
stellar initial mass function (IMF) is still debated. Particularly crucial is
the question of what sets the peak of the distribution. To investigate this
issue we perform high resolution numerical simulations with radiative feedback
exploring in particular the role of the stellar and accretion luminosities. We
also perform simulations with a simple effective equation of state (eos) and we
investigate 1000 solar mass clumps having respectively 0.1 and 0.4 pc of
initial radii. We found that most runs, both with radiative transfer or an eos,
present similar mass spectra with a peak broadly located around 0.3-0.5
M$_\odot$ and a powerlaw-like mass distribution at higher masses. However, when
accretion luminosity is accounted for, the resulting mass spectrum of the most
compact clump tends to be moderately top-heavy. The effect remains limited for
the less compact one, which overall remains colder. Our results support the
idea that rather than the radiative stellar feedback, this is the transition
from the isothermal to the adiabatic regime, which occurs at a gas density of
about 10$^{10}$ cm$^{-3}$, that is responsible for setting the peak of the
initial mass function. This stems for the fact that $i)$ extremely compact
clumps for which the accretion luminosity has a significant influence are very
rare and $ii)$ because of the luminosity problem, which indicates that the
effective accretion luminosity is likely weaker than expected.
