  Molecular fragment or embedding methods are powerful techniques for
overcoming scalability limitations in electronic structure theory by dividing
large molecular systems into individual units that are small enough to be
treated using standard techniques. Our group has developed a linear-scaling
coupled-cluster based fragment effective field method, molecular cluster
perturbation theory (MCPT), that provides a systematically improvable
description of the electronic wavefunction and is scalable to thousands of
atoms using the Aces4 massively parallel software package. We will present a
systematic computational benchmarking study of small organic molecule solvation
free energies obtained from MCPT using various perturbation corrections using
experimental and large-scale ab initio reference values. We will also present
recent work on the relative crystal lattice polymorph energies of several
molecular crystals where the roles of many-body dispersion and intermol ecular
electron exchange will be investigated. The molecular solvent and crystal
systems presented here are carefully chosen so as to provide a robust test set
for the fragment electronic structure community.
