  We study the photoevaporation of Jeans-unstable molecular clumps by isotropic
FUV (6 eV $< {\rm h}\nu$ < 13.6 eV) radiation, through 3D radiative transfer
hydrodynamical simulations implementing a non-equilibrium chemical network that
includes the formation and dissociation of H$_2$. We run a set of simulations
considering different clump masses ($M=10-200$ M$_\odot$) and impinging fluxes
($G_0=2\times 10^3-8\times 10^4$ in Habing units). In the initial phase, the
radiation sweeps the clump as an R-type dissociation front, reducing the H$_2$
mass by a factor 40-90%. Then, a weak ($\mathcal{M}\simeq 2$) shock develops
and travels towards the centre of the clump, which collapses while loosing mass
from its surface. All considered clumps remain gravitationally unstable even if
radiation rips off most of the clump mass, showing that external FUV radiation
is not able to stop clump collapse. However, the FUV intensity regulates the
final H$_2$ mass available for star formation: for example, for $G_0 < 10^4$
more than 10% of the initial clump mass survives. Finally, for massive clumps
($\sim 100$ M$_\odot$) the H$_2$ mass increases by 25-50% during the collapse,
mostly because of the rapid density growth that implies a more efficient H$_2$
self-shielding.
