On the transferability of pure electron correlation of quantum topologically defined molecular fragments at the CCSD(T) level.

The Interacting Quantum Atoms (IQA) method offers a route to calculate, directly from the two-particle-density-matrix, electron correlation energies for a quantum topological atom interacting with itself and with other atoms in a given system. Here, we corroborate the energetic transferability, at CCSD(T) level, of a variety of functional groups as they appear in both small organic molecules and water-containing complexes: methyl, amine, hydroxyl, water, carbonyl, carboxyl, amide, methylene and triple bonds. The dependence of these atomic correlation energies upon variation of a dihedral angle is very small (0.1 kJ/mol), and that upon variation in bond length and angle reasonably small (units of kJ/mol for molecular dynamics-induced changes and tens of kJ/mol for extreme, manual changes). Overall, these findings give the green light for the machine learning of these physically-informed atomic properties. The beneficiary is the force field FFLUX, which will then be able to make predictions on dispersion effects, totally in line with the other types of IQA energy contributions that underpin it. Hence the familiar dispersion schemes that are typically bolted on become unnecessary.