The second order derivative term vanishes when adopting a linear approximation; it varies slowly with the normal mode coordinates . When the first-order derivative of the initial electronic state displays minimal variance with respect to Q j , the Condon approximation, or the course adiabatic approximation, can be applied . Further, cross terms between different vibrational normal modes can be neglected by assuming a single promoting mode . This is the formal casting of the Born-Oppenheimer coupling . The first order derivative term Equation , is known to have 3 problems .