Hessian matrix potential energy surface, vibrational

Scuseria and Schaefer [202a] followed up some earlier work by attempting to determine whether the open trimer of HF represents a true minimum in the potential energy surface. Within the context of their double-C basis set, it was found that the cis open trimer is indeed a minimum (Hessian matrix has all positive eigenvalues), whereas the trans structure is a transition state. However, the situation is decidedly different when polarization functions are added to the basis set wherein the trans geometry becomes a transition state and the cis type does not correspond to a stationary point of any order. Nor does relaxing the fully planar restraint lead to a cis stationary point. Vibrational frequencies and intensities were reported for the stationary points. 

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