For the isolated 87 Rb atom the transition energy from the 5 2 S ground state to the 5 2 P excited state is predicted to be at 1.488 eV at the CASPT2/TZP level of theory. This can be compared with the experimental values of 1.560 and 1.589 eV for the D 1 (5 2 S 1/2 to 5 2 P 1/2 ) and D 2 (5 2 S 1/2 to 5 2 P 3/2 ) transitions, respectively. Taking spin-orbit coupling into account for an isolated atom of 87 Rb using SS-CASPT2/TZP energies, the transition energy for D 1 is at 1.476 eV with the D 2 transition energy at 1.494 eV. One conformation for each of the 87 Rb-coat-molecules is considered and are the lowest energy conformers, with no imaginary harmonic vibrational frequencies, of those we found at the MP2/def2-TZVP level for 87 Rb-ethane (two conformers), 87 Rb-ethene (three conformers) and 87 Rb-MeTS (three conformers). Figure displays their optimised structures and their Cartesian coordinates are available in the Supplementary Information. For each system constrained potential energy scans were performed in 0.2 Å intervals along the coordinate between 87 Rb and the coat molecule. The distance to the 87 Rb atom is from the CC bond midpoint in ethane and ethene, and the plane of the three Cl atoms in MeTS. The electronic states of 87 Rb, in the environment of the coat molecule, are labelled as 1 2 A for the ground state (5 2 S) and 2 2 A, 3 2 A and 4 2 A for the three excited states (5 2 P), one for each Cartesian component or azimuthal quantum number of the 5p orbital and in ascending order of energy.