Figure : Potential energy curves of the three lowest excited singlet states of acetylene (the states that become 1 Σ - u and two components of 1 ∆ u at linear geometries) computed with XMS-CASPT2, XMS-PDFT, CMS-PDFT, MC-PDFT, HMC-PDFT, L-PDFT, and HL-PDFT. The zero of energy is taken as the energy of the first excited singlet state at θ = 179 • . The ground-state potential energy curves for these methods are plotted in Figure .