The Limit of Mechanical Stability in Quantum Crystals: A Diffusion Monte   Carlo Study of Solid 4He
  We present a first-principles study of the energy and elastic properties of solid helium at pressures below the range in which is energetically stable. We find that the limit of mechanical stability in hcp 4He is $P_{s}$ = -33.82 bar, which lies significantly below the spinodal pressure found in the liquid phase (i.e., -9.6 bar). Furthermore, we show that the pressure variation of the transverse and longitudinal sound velocities close to $P_{s}$ do not follow a power law of the form $\propto \left( P - P_{s} \right)^{\gamma}$, in contrast to what is observed on the fluid. 
Claudio Cazorla and Jordi Boronat
cond-mat.other cond-mat.quant-gas