Stabilization Method as a Tool for Electronic State Spectroscopy

We propose a new systematic approach to distinguish valence and diffuse electronically excited states of polyatomic molecules. This method applies the stabilization technique of Hazi and Taylor [<i>Phys. Rev. A</i> <b>1970</b>, 1 (1109)] to neutral excited states, using basis set diffuseness as the stabilization parameter. By monitoring how excited states behave under basis set contraction, one can differentiate valence states from Rydberg and mixed valence-Rydberg states, reducing the arbitrariness of previous strategies. Valence states remain stable, whereas the energies of Rydberg and mixed states vary. This approach is compatible with any electronic structure method. To illustrate its applicability, we characterize the singlet excited states of CCl₄, HCOOH, and 2-chlorotoluene. This provides a valuable tool for investigating the nature of the electronically excited states.