Having determined the harmonic vibrational frequencies and normal modes of vibration from the well-established eigenanalysis of the molecular Hessian,20 it is possible to make anharmonic corrections to fundamental vibrational frequencies and frequencies corresponding to combination or overtone excitations of the normal modes by a second-order perturbational approach, where the resulting expressions involve the cubic and quartic force constants and Coriolis vibration–rotation coupling constants. In the VPT2 approach,23,35,36 the corrected fundamental vibrational frequencies ν i1, first overtone frequencies ν i2 and first combination frequencies ν i1j1 are given as, respectively24 25νi2 = 2νi1 + 2Xii, 26νi1j1 = νi1 + νj1 + Xij,where the diagonal and off-diagonal correction terms X ii and X ij are given by27and28where Ω ijk is defined as29Ωijk = (ωi + ωj + ωk)·(–ωi + ωj + ωk)·(ωi – ωj + ωk)·(ωi + ωj – ωk). In the above expressions, ω i denotes a harmonic fundamental frequency, φ ijk and φ ijkl are cubic and quartic force constants, respectively, B α is the rotational constant for axis α, and ζ α ij is a Coriolis coupling constant.