Basis Set, Correlation, and Anharmonicity

The level of theory necessary to correctly model vibrational spectra is a lively topic in modern ab initio chemistry and has been discussed on a number of occasions.The reader is referred to some of these papers for discussion of calculated spectra in single molecules. The present discussion focuses on application to H-bonded complexes. 

Botschwina ° has demonstrated the importance of anharmonic effects to accurate evaluation of the HF stretching frequency in HCN —HF. He used a large doubly polarized basis set and included correlation via the CEPA-1 approach. Anharmonicity reduced the HF stretching frequency from 4294 to 4053 cm. The red shift of this frequency, i.e., the change in the complex vs. the isolated HF molecule, increased from 164 to 242 cm when anharmonicity was included. 

Bouteiller et al. considered complexes involving a hydrogen halide and an amine with regard to effects of both correlation and anharmonicity upon vibra- 

Anharmonicity was considered in a different context by Bouteiller et al. to try to explain the band shape of the V( x transition in FH—0(CH3)2. The authors calculated a detailed ab initio two dimensional energy surface, corrected for basis set superposition error, This surface was then fit to a fourth-order polynomial expression in terms of both r(HF) and R(F—0) of the form 

A variational procedure was applied to determine the fundamentals and first overtones for this potential. Since the surface was computed at the SCF level, it was not surprising to find the fundamental V. -h frequency overestimated the experimental quantity by 8%. Most encouraging was the agreement between the calculated (174 cm ) and experimental V[,-...(3 (185 + 10 cm ) fundamental. 

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