Distortion effects and clustering of isotopic impurities in solid molecular para-hydrogen from variational Monte Carlo simulations with shadow wave functions

We employed a fully optimized shadow wave function in combination with variational Monte Carlo techniques to investigate the properties of HD molecules and molecular ortho-deuterium (o-D-2) in bulk solid para-hydrogen (p-H-2). Calculations were performed for different concentrations of impurities ranging from about 1% to 25% at the equilibrium density for the para-hydrogen crystal. By computing the excess energy both for clustered and isolated impurities, we tried to determine a limit for the solubility of HD and o-D-2 in p-H-2.