= Transverse electric-dipole pair energy
The point-dipole <energy> is $[\mathbf p_1\cdot\mathbf p_2-3(\mathbf p_1\cdot\hat r)(\mathbf p_2\cdot\hat r)]/(4\pi\epsilon_0r^3)$. If the separation is perpendicular to the first dipole, the second term vanishes. Parallel dipoles then have positive <energy> $p_1p_2/(4\pi\epsilon_0r^3)$, while mutually perpendicular dipoles give zero in this approximation. Real molecular interactions can include multipoles, induction and short-range contributions beyond this point-dipole term.
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