Force constants, open-shell molecules

Also as already noted above, taking advantage of molecular symmetry can provide very large savings in time. However, structures optimized under the constraints of symmetry should always be checked by computation of force constants to verify tlierr nature as stationary points on die full PES. Additionally, it is typically worthwhile to verify that open-shell wave functions obtained for symmetric molecules are stable with respect to orbital changes that would generate other electronic states. 

A correlation between the CO bond properties in the closed-shell molecules (single and double bonds) was proposed to estimate the bond lengths and stretching frequencies of open-shell phenoxyl radicals. Nevertheless, while it is possible to estimate the CO force constants using the Badger-type relations, it is difficult to relate them to the experimental frequencies that do not represent the stretching of a single bond. 

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