Anharmonic force field for methanol

An ab initio quartic anharmonic force field for methanol has been calculate d at the equilibrium position using the CCSD(T) method for the structure an d the harmonic potential energy surface, and the MP4(SDQ) method for the an harmonic part of the surface. A triple zeta basis set was employed with sym metrized curvilinear internal valence coordinates in all calculations. The internal coordinate force field constants have been transformed into force constants in the dimensionless normal coordinate representation for various isotopomers. Vibrational term values for CH3OH, CH3OD, CD3OH, and CD3OD ha ve been obtained using second order perturbation theory. Particular care ha s been devoted to the inclusion of Fermi resonance interactions between dif ferent vibrational states. A good accuracy has been achieved in the calcula tion of the fundamentals for all the isotopomers, the mean absolute error b eing 5.8 cm(1).