The MS-CASPT2 calculated ground-state energies, VEEs, VDEs and ADEs of conformers 1a–1d, relative to the ground electronic state of conformer 1a, are listed in Table 1 and plotted in Fig. 3. Notably, although the VDE is more or less constant for all conformers, the ADE is lowered by approximately 0.7 eV for 1b compared to 1a.