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tokens: Virial equilibrium (PVE), instead of the usual SVE. The implication, being a single magnitude of the external pressure, P ext , may be insufficient to explain the variations reported by Heyer etal. ( 2009 ). Instead, their data could be reconciled only if different clouds in the sample experienced different magnitudes of pressure in the range 10 4 10 7 K cm 3 . Equivalently, it meant that the sizelinewidth relation would be modified by the magnitude of external pressure, P ext , experienced by a GMC. In an earlier work, Ballesteros-Paredes ( 2006 ) strongly argued against GMCs obeying the SVE because: (i) clouds being the consequence of turbulent fragmentation in the interstellar medium (ISM), the fragmentation process must necessarily induce a flux of mass, momentum and energy between clouds and the ISM, and (ii) clouds often exhibit asymmetries in their respective line-profiles, which is inconsistent with clouds obeying the SVE. Instead, the Larsons scaling relations should, at best, be viewed as evidence for energy-equipartition. The possibility of bound, turbulence-supported clouds as suggested by Krumholz McKee ( 2005 ) is difficult to reconcile, for it is unlikely that there will be no redistribution of mass within such clouds over their lifetime (Ballesteros-Paredes 2006 ). Gas dynamics within a typical GMC is likely governed by collisions between smaller clumps within them (e.g. Anathpindika 2009a , b and references therein), and/or feedback from existing stellar populations (see e.g. Mac Low Klessen 2004 and references therein). In a more recent work, Ballesteros-Paredes etal. ( 2011 ) argued that the variations in the sizelinewidth relation reported by Heyer etal. ( 2009 ) is actually consistent with the scenario of hierarchical fragmentation in which a cloud does not collapse as a whole, but only isolated pockets in it collapse to form stars. In this paradigm, although turbulence in the ISM possibly plays a role in assembling the cloud, post its formation, when the contribution due to self-gravity becomes significant, the cloud no longer needs to be in equilibrium with the external medium. It is therefore unnecessary to resort to mechanisms that can actually hold clouds with complex geometries and density distributions in approximate equilibrium (see also Burkert Hartmann 2004 ). Simulations of unbound clouds leading to inefficient star formation were discussed by e.g. Clark etal. ( 2005 ) and Clark, Bonnell Klessen ( 2008 ). The virial state of GMCs has also been interrogated numerically. Dobbs etal. ( 2008 ) and Tasker Tan ( 2009 ), for instance, reported that clouds forming in a galactic disc typically have a virial coefficient between 0.2 and 10. Furthermore, Dobbs etal. ( 2011 ) argued that intercloud collisions in the disc of a galaxy was a viable mechanism to render clouds unbound. Alternatively, several other authors examined the formation of clouds out of collisions between large-scale flows (e.g. Vzquez-Semadeni, Passot