Vibrational many-body methods and Born–Oppenheimer approximation

We apply vibrational analogues of self-consisitent field, many-body perturbation, and configuration-interaction methods to molecules as well as to polymers and solids in close collaboration with Dr. Kiyoshi Yagi. Combining these methods with accurate electron-correlation methods for PES, we have determined the most accurate quartic force field of CO₂, giving its Fermi doublet frequencies within a few cm^–1 of the observed.

We have also challenged 80-year-old wisdom of Born and Oppenheimer concerning how the errors in the BO approximation scale with respect to the mass ratio of light (m) to heavy (M) particles. We have concluded that the correct scaling should be (m/M)¹ for BO or (m/M)^3/2 and (m/M)² when diagonal BO corrections are applied perturbatively and variationally, unlike (m/M)^5/4 as Born and Oppenheimer suggested.