  We introduce and employ two QM:QM schemes (a quantum mechanical method
embedded into another quantum mechanical method) and report their performance
for the X23 set of molecular crystals. We furthermore present the theory to
calculate the stress tensors necessary for the computation of optimized cell
volumes of molecular crystals and compare all results to those obtained with
various density functionals and more approximate methods. Our QM:QM
calculations with PBE0:PBE+D3, PBE0:PBE+MBD, and B3LYP:BLYP+D3 yield at a
reduced computational cost lattice energy errors close to the ones of the
parent hybrid density functional method, whereas for cell volumes, the errors
of the QM:QM scheme methods are in between the GGA and hybrid functionals.
