Within the Born-Oppenheimer and Condon approximations, FCFs determine vibrational branching of the electronic transition. The most desired property for optical cycling is a diagonal FCF matrix-so that there is no vibrational branching during electronic decay. Because repumping schemes can mitigate the effect of leaking population, cumulative FCFsthe sum of the first few FCFs with fixed ν I -are also valuable in evaluating quality of an OCC. In diatomic molecules, FCFs are determined by the change in the position of the minimum and the curvature of the potential energy curves of the ground and excited states. The position of the minimum (i.e., equilibrium bond length) depends on the bonding pattern, determined by the shape of occupied molecular orbitals. Hence, the change in electron density upon excitation provides a visual cue whether the bonding pattern changes and whether or not one may expect diagonal FCF-i.e., large changes in the inter-atomic charge distribution are expected to give rise to a significant bond length change and, consequently, non-diagonal FCFs.