Representation of a chemical system of interacting species: Ions and biomolecules in solution, as an example

A realistic representation of an ensemble of interacting molecules (or atoms, or ions) requires a description that includes the following parameters: the electronic energy, E, the electronic density, ρ, the temperature, T, the free energy, ΔG, and the entropy, ΔS. It is, therefore, necessary to consider as minimal requirement both quantum chemistry and statistical thermodynamics. In this work we describe how to operationally link the two fields, traditionally somewhat disconnected. An application of the method is given considering the problem of water molecules arranged around a molecule of water, ion‐pairs, single ions, amino acids, bases of DNA and other biomolecules. The importance to obtain simple transferable interaction potential, and a methodology to obtain such potentials is discussed. An energy partitioning technique is presented in order to obtain pair‐potentials from ab‐initio computations. The technique allows to propose a quantum chemical definition of atoms and molecules.