Density Fitted Fragment Embedding – Principles and Applications

We demonstrate how a density fitted fragment embedding framework can be constructed using both non-overlapping fragments and overlapping fragments to perform energy evaluations. We then implement the framework to first perform energy calculations on water clusters using a polarised 3-zeta basis set to demonstrate the observed scaling of the algorithm, which is O(N^2.61) and is capped at O(N^3.57) in the large fragment limit. We then perform numerical structural optimisations on cyclic water and hydrogen fluoride clusters using this theory. The optimised structures and the binding energies are then compared with results using the corresponding all-electron solver, which is CCSD in this work. It turns out that BE1-DF-CCSD is able to recover at least 94% of the hydrogen bonding energy in both water and hydrogen fluoride clusters. This work therefore provides foundations for the efficient generation and quality of fragment embedding energy data in large, weakly interacting systems.