Figure 1.

Deviations of harmonic ω and anharmonic ν wavenumbers from experimental values (the origin of the y axis) for acetylene (in cm⁻¹). Experimental values are reported in the x axis at the bottom and the corresponding assignment at the top. The series of four values for each anharmonic frequency stands for, from left to right, VPT2, DCPT2, HDCPT2, DSPT2, and HDSPT2 treatments for possibly resonant terms. Computational methods: MP2 and B2PLYP with AVTZ basis set and B3LYP with SNSD. CCSD(T)/A'CVQZ harmonic and anharmonic frequencies from Table 5 of Ref.163. In the hybrid method, the harmonic frequencies are from CCSD(T)/A'CVQZ and the anharmonic force-field from B2PLYP/AVTZ calculations. Experimental values are taken from Ref.163 for fundamental frequencies, and from Ref.59 for overtones and combination bands. MAE stands for mean absolute error.