Thus, the changes in the total wavefunction with time will have a direct influence on the nuclear motion. Often, to decrease computational costs, the electronic "back reaction" on the nuclear motion is neglected, and this approximation is known as the classical path approximation (CPA). As such, within the CPA, the nuclear motion is not affected by the electron dynamics. Nonetheless, the electron dynamics still depends on the nuclear coordinates (due to the parametric dependence of the adiabatic eigenvectors on the nuclear coordinates at each time-step), and the CPA is generally valid when the ground and excited state PESs differ slightly.