Application of the instanton method for analyzing tunneling splitting spectra of nonrigid molecular systems

General formalism and basic ideas of the instanton method are described. The potentialities of the method are demonstrated by calculating the tunneling splitting of the ground vibrational level for nonrigid molecules with two and three equivalent minima on their potential-energy surface. Some advantages of the application of the instanton method as compared to the well-known semiclassical approximation for describing the spectra of tunneling splitting of a nonrigid molecular system are proved.