A formula giving an expression for the induced-dipole-induced-dipole interaction between molecules (called the dispersion interaction or London interaction). The London formula for the interaction energy V is given by V = –C/r⁶, where C = ⅔α‎′₁α‎′₂I₁I₂/(I₁+I₂). Here α‎′₁ and α‎′₂ are the polarizability volumes of molecule 1 and 2 respectively, I₁ and I₂ are the ionization energies of molecules 1 and 2 respectively, and r is the distance between the molecules. The London formula is named after Fritz London (1900–54), who derived it in 1930. The interaction described by the London formula is usually the dominant term in intermolecular forces (unless hydrogen bonds are present).