Fig. 4. The S₁ relax-scan potential energy curves (Ex_(4,6)) as a function of the O–H coordinates of the water dimer obtained from CASPT2(4,6) calculations. The S₀ potential energy curves (Gr_(4,6)) were calculated at the same geometries. The dash-line curves are the S₀ and S₁ energies obtained from CASPT2(16,12) calculations, Gr_(16,12) and Ex_(16,12), respectively. The relative energies (ΔE) are with respect to structure G2-[1]. The value of the LUMO isosurfaces is 0.065. Energies and distances are in kJ mol⁻¹ and Å, respectively. The energy release (ΔE^Rel) and energy barrier (ΔE^‡) are in kJ mol⁻¹; ‡ = transition structure; (I) = photoexcitation; (II) and (III) = photoionization.