Integral Hellmann-Feynman investigations oftrans bent acetylene

The integral Hellmann-Feynman (IHF) theorem has been applied to various wavefunctions representing the¹ A _u state of acetylene with a view to testing traditional explanations of the excited state geometry. When LCAOSCF wavefunctions are used, the electronic energy changes associated with the individual corresponding orbitals (CMG's) are in sympathy with the trend in orbital (i.e. MO) energies suggested by Walsh. However, when LMO wavefunctions based on hybrid AO's are employed, the IHF results are against all experience; and imply that a change in hybridisation, fromsp tosp ², is not a viable model for the change in geometry.