Alchemical variations of intermolecular energies according to molecular grand-canonical ensemble density functional theory

Molecular grand-canonical density functional theory [J. Chem. Phys. 2006, 125, 154104] is employed for the alchemical variation of intermolecular energies due to changes in the chemical composition of small molecules. We investigate the interaction of a fixed binding target, formic acid, with a restricted chemical space, corresponding to an isoelectronic 10-proton system which includes molecules such as CH4, NH3, H2O, and HF. Differential expressions involving the nuclear chemical potential are derived, numerically evaluated, tested with respect to finite difference results, and discussed regarding their suitability as gradients of the intermolecular energy with respect to compositional variations.