It is shown that the presence of even a small amount of spin contamination in UHF wavefunctions can cause significant errors in calculated vibrational frequencies at the UMP2 level. The magnitude of the error is not related to the degree of spin contamination, as measured by the expectation value of the S operator, but to the geometrical derivative of 〈S〉. The PMP2 method gives some improvement over UMP2 values, although the errors are still significant. The recently introduced AUMP2 method, which annihilates self-consistently the next-higher spin contamination, is shown to give results in much better agreement with experimental data and comparable to those obtained with spin-restricted wavefunctions for singlet molecules. The calculations also allow an estimate of the unknown structure of the ClOO molecule to be made.
