Gauge theory and quasidiabatic states in molecular physics

Following previous works [Moody et al., Phys. Rev. Lett. 56, 893 (1986) and Zygelman, Phys. Lett. A 125, 476 (1987)], we review in some detail the gauge theoretical form of the Born–Oppenheimer description of molecules. The derivative couplings which describe nonadiabatic effects play the role of gauge potentials. Applying the gauge theoretical formalism (well known in particle physics), we rederive a theorem concerning the nonexistence of strictly diabatic states. Further we propose the Lorentz gauge condition to define quasidiabatic states. We show that the recently proposed block diagonalization method to obtain quasidiabatic states leads automatically to the Lorentz gauge locally.