When using a given basis set, both energy and this quantity are multiplied by a dimensionless scaling factor usually close to 1 to account for a term called ZPVE. For a transition state, this quantity takes on an imaginary value since it corresponds to a negative eigenvalue of the mass-weighted Hessian of a Born-Oppenheimer surface. Olfactory receptors identify molecules by their value for this quantity, according to the controversial ■END■