 Eckart conditions

The Eckart conditions, named after Carl Eckart, sometimes referred to as Sayvetz conditions, simplify the nuclear motion (rovibrational) Schrödinger equation that arises in the second step of the Born–Oppenheimer approximation.The Eckart conditions allow to a large extent the separation of the external (rotation and translation) motions from the internal (vibration) motions.