  How molecular clouds fragment and create the dense structures which go on to
form stars is an open question. We investigate the relative importance of
different energy terms (kinetic, thermal, magnetic, and gravity - both
self-gravity and tidal forces) for the formation and evolution of molecular
clouds and their sub-structures based on the SILCC-Zoom simulations. These
simulations follow the self-consistent formation of cold molecular clouds down
to scales of 0.1 pc from the diffuse supernova-driven interstellar medium in a
stratified galactic disc. We study the time evolution of seven molecular clouds
(five with magnetic fields and two without) for 1.5-2 Myr. Using a dendrogram,
we identify hierarchical 3D sub-structures inside the clouds with the aim to
understand their dynamics and distinguish between the theories of
gravo-turbulent fragmentation and global hierarchical collapse. The virial
analysis shows that the dense gas is indeed dominated by the interplay of
gravity and turbulence, while magnetic fields and thermal pressure are only
important for fluffy, atomic structures. Over time, gravitationally bound
sub-structures emerge from a marginally bound medium (viral ratio $1 \leq
\alpha_{\rm vir}^{\rm vol} <2$) as a result of large-scale supernova-driven
inflows rather than global collapse. A detailed tidal analysis shows that the
tidal tensor is highly anisotropic. Yet the tidal forces are generally not
strong enough to disrupt either large-scale or dense sub-structures but cause
their deformation. By comparing tidal and crossing time scales, we find that
tidal forces do not seem to be the main driver of turbulence within the
molecular clouds.
