FIG. 3.

Cartesian coordinate system for the AB dimer. Monomer EDTMs, μ^A and μ^B, are separated by the distance R_AB and twist angle ${\varphi }_{\mathit{A}\mathit{B}}$. The monomer EDTMs are assumed to be perpendicular with respect to the z-axis. The symmetric exciton μ^A + μ^B (shown in blue) and the anti-symmetric exciton μ^A − μ^B (shown in red) are each oriented parallel to the x- and y-axis, respectively. The x-axis is an axis of C₂ symmetry, as indicated. (The y and z axes are similarly C₂ symmetry elements.) Note that the point dipole-dipole coupling strength $J\propto \cos {\varphi }_{\mathit{A}\mathit{B}}$ [see Eq. (6) below] undergoes a sign inversion at ${\varphi }_{\mathit{A}\mathit{B}}$ = 90°.