Atoms and electron‐pair bonds in a molecule: An approach within the context of extended geminal models

Within the framework of extended geminal models, the meaning of the electron‐pair bond concept is substantiated. If there exist two‐centered electron‐pair bonds in a molecule, it is demonstrated that the molecule can be partitioned into neutral fragments. The total electronic energy can then be written as a sum of intra‐ and interfragment energies. If the electron‐pair bonds are only on the two‐center type, the advocated procedure leads to the definition of an atom in a molecule. As test cases, the bonding in H 2 and HF, the difference in equilibrium geometries of H 2 O and H 2 S, and the origin of the ethane rotational barrier are analyzed.