Quantum mechanical explanation of intermolecular interactions

In the natural sciences, an intermolecular force is an attraction between two molecules or atoms. They occur from either momentary interactions between molecules (the London dispersion force) or permanent electrostatic attractions between dipoles. They can be explained using a simple phenomenological approach (see intermolecular force), or using a quantum mechanical approach.

Hydrogen bonding, dipole–dipole interactions, and London (Van der Waals) forces are most naturally accounted for by Rayleigh–Schrödinger perturbation theory (RS-PT). In this theory—applied to two monomers A and B—one uses as unperturbed Hamiltonian the sum of two monomer Hamiltonians, ${\displaystyle H^{(0)}\equiv H^{A}+H^{B}}$

In the present case the unperturbed states are products ${\displaystyle \Phi _{n}^{A}\Phi _{m}^{B}\quad }$ with ${\displaystyle \quad H^{A}\Phi _{n}^{A}=E_{n}^{A}\Phi _{n}^{A}\quad }$ and ${\displaystyle \quad H^{B}\Phi _{m}^{B}=E_{m}^{B}\Phi _{m}^{B}}$

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