# General Formalism

A central potential is a potential that depends only on the absolute value of the distance away from the potential's center. A central potential is rotationally invariant. We may use these properties to reduce this otherwise three-dimensional problem to an effective one-dimensional problem. The general form of the Hamiltonian for a particle immersed in such a potential is

$\hat{H}=\frac{\hat{p}^2}{2m}+V(|\hat{r}|).$

Due to rotational symmetry, $[\hat{H},\hat{L}_z]=0\!$ and $[\hat{H},\hat{L}^2]=0.\!$ This allows us to find a complete set of states that are simultaneous eigenstates of $\hat{H},\!$ $\hat{L}_z,\!$ and $\hat{L}^2.\!$ We will label these eigenstates as $|n,l,m\rangle,\!$ where $l\!$ and $m\!$ are the orbital and magnetic quantum numbers, as defined in the previous chapter, and $n\!$ represents the quantum numbers that define the radial dependence of the wave function; this is the only part of the state that depends on the exact form of the potential, as we will see shortly.

Let us now write the Schrödinger equation for this system and solve for the angular dependence of the wave function, thus reducing the problem to an effective one-dimensional problem. The equation is

$-\frac{\hbar^2}{2m}\nabla^2\psi+V(r)\psi=E\psi.$

The Laplacian in spherical coordinates may be written as

$\nabla^2=\frac{1}{r}\frac{\partial^2}{\partial r^2}r-\frac{\hat{\mathbf{L}}^2}{\hbar^2 r^2},$

so that the equation becomes

$\left(-\frac{\hbar^2}{2m}\frac{1}{r}\frac{\partial^2}{\partial r^2}r+\frac{\hat{\mathbf{L}}^2}{2mr^2}+V(r)\right)\psi=E\psi.$

We already know the eigenfunctions of $\frac{\hat{\mathbf{L}}^2}{2mr^2}$ from the previous chapter, and thus the entire angular dependence of the wave function. We may therefore use separation of variables and write $\psi(r,\theta,\phi)=f_l(r)Y_l^m(\theta,\phi),\!$ where $Y_l^m(\theta,\phi)\!$ are the spherical harmonics. Substituting this into the Schrödinger equation, we obtain

$\left(-\frac{\hbar^2}{2m}\frac{1}{r}\frac{d^2}{dr^2}r+\frac{\hbar^2 l(l+1)}{2mr^2}+V(r)\right)f_l(r)=Ef_l(r).$

The term $\frac{\hbar^2 l(l+1)}{2mr^2}$ is referred to as the centrifugal barrier, which is associated with the motion of the particle. The classical analogue is the term, $\frac{l^2}{2mr^2},$ that arises in treating central forces classically. The centrifugal barrier prevents the particle from reaching the center of force, causing the wave function to vanish at this point.

Now, if we let $u_l(r)=rf_l(r)\!$, we finally arrive at the effective one-dimensional Schrödinger equation for the radial dependence of the wave function, $\left(-\frac{\hbar^2}{2m}\frac{d^2}{dr^2}+\frac{\hbar^2 l(l+1)}{2mr^2}+V(r)\right)u_l(r)=Eu_l(r).$

Due to the boundary condition that $f_l(r)\!$ must be finite the origin, $u_l(r)\!$ must vanish.

In many cases, looking at the asymptotic behavior of $u_l(r)\!$ can be quite helpful, as we will see in later sections.

## Nomenclature

Historically, the first four (previously five) values of $l\!$ have taken on names, and additional values of $l\!$ are referred to alphabetically:

$\begin{cases} l = 0 & \mbox{s-wave (sharp)}\\ l = 1 & \mbox{p-wave (principal)}\\ l = 2 & \mbox{d-wave (diffuse)}\\ l = 3 & \mbox{f-wave (fundamental)}\\ l = 4 & \mbox{g-wave (previously called t-wave for thick)}\\ l = 5 & \mbox{h-wave}\\ \end{cases}$