with K denoting the orthogonal cartesian-to-normal modes conversion matrix and {R 0,ν } the cartesian coordinates at the reference geometry. {E i } are the electronic state energies at a reference geometry q = q 0 and f i (q ν ) is a harmonic function for modes 10,12,18,20,21 and a Morse function for modes 24,25,26 q 12 q 18 q 20 q 21 q 24 q 25 q 26 ω 734