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# 8: Approximate Methods

So far, we have concentrated on problems that were analytically solvable, such as the simple harmonic oscillator, the hydrogen atom, and square well type potentials. In fact, we shall soon be confronted with situations where an exact analytic solution is unknown: more general potentials, or atoms with more than one electron. To make progress in these cases, we need approximation methods. The best known method is perturbation theory, which has proved highly successful over a wide range of problems (but by no means all).

• 8.1: Variational Methods
In this module, the variational method is introduced. The variational method works best for the ground state, and in some circumstances (to be described below) for some other low lying states.
• 8.2: The WKB Approximation
The WKB (Wentzel, Kramers, Brillouin) approximation is, in sense to be made clear below, a quasi-classical method for solving the one-dimensional (and effectively one-dimensional, such as radial) time-independent Schrödinger equation.
• 8.3: Note on the WKB Connection Formula
For a particle trapped in a (one-dimensional) potential well, classically the particle would bounce back and forth between the two turning points where its kinetic energy vanishes. In the quantum case, these are precisely the points where the wavelength becomes infinite, so the WKB solution fails.

Thumbnail: Two (or more) wave functions are mixed by linear combination. The coefficients c1, c2 determine the weight each of them is given. The optimum coefficients are found by searching for minima in the potential landscape spanned by c1 and c2. Image used with permission (CC BY-SA 3.0; Rudolf Winter at Aberystwyth University).