The mass of an electron is 1860 times less than that of a proton. This is a crucial difference since it permits us to treat the electrons as a wave function while treating the nuclear positions as fixed. In effect, the nuclei are not being treated using the wave-like properties. If they were then we would need to solve two simultaneous wave equations in order to describe even the simplest atom. The mass difference also leads to a difference in time scale for motion, which is the fundamental physical reason that the aproximation holds. In practice this means that we solve for the energies of molecules at fixed nuclear positions, R. R here can represent a set of coordinates. The potential energy U(R) can be determined for a number of different values of R in order to determine at potential energy profile or even a potential energy surface. We can also use this concept to determine the minimum energy by geometry optimization. This is discussed in detail in the next chapter when we consider the calculation of normal modes.
