Fig. 3. The S₁ relax-scan potential energy curve (Ex_(4,4)) with respect to one of the O–H coordinates of the water monomer (structure E1-[2] in Fig. 1) obtained from CASPT2(4,4) calculations. The energies on the S₀ potential energy curve (Gr_(4,4)) were calculated at the same geometries. The relative energies (ΔE) are with respect to the total energy of structure G1-[1]. The dash-line curves are the S₀ and S₁ energies obtained from CASPT2(6,5) calculations, Gr_(6,5) and Ex_(6,5), respectively. The value of the LUMO isosurfaces is 0.065. Energies, distances and angles are in kJ mol⁻¹, Å and degree, respectively. ΔE^‡ = Energy barrier in kJ mol⁻¹.