# Double-well Schrödinger eigenstates

Nick Trefethen, November 2010

(Chebfun Example ode-eig/DoubleWell.m)

A well-known problem in quantum mechanics is the calculation of eigenstates of a potential with the shape of a 'double well'. Specifically, consider a potential function V(x) defined on [-1,1] by

`  V(x) = 1.5 for x in [-.2,.3],  0 otherwise.`

We seek eigenmodes of the steady-state Shrödinger equation associated with this potential, specifically, functions u(x) satisfying

`   -0.007u"(x) + V(x)*u(x) = lam*u(x),    u(-1) = u(1) = 0.`

for some constant lam.

We can sketch the potential like this:

```LW = 'linewidth'; hold off
plot([-1 -1 -.2 -.2 .3 .3 1 1],[3.3 0 0 1.5 1.5 0 0 3.3],'k',LW,3)
axis([-1.1 1.1 -.05 3.3]), axis off, hold on
``` Let's compute the first 12 eigenvalues and eigenfunctions:

```tic
x = chebfun('x');
V = 1.5*(abs(x-0.05)<0.25);
L = chebop(-1,1);
L.op = @(x,u) -0.007*diff(u,2) + V.*u;
L.bc = 0;
neigs = 12;
[EV,D] = eigs(L,neigs);
disp(diag(D)), toc
```
```    0.0915
0.1168
0.3639
0.4632
0.8089
1.0211
1.3908
1.6526
1.8712
2.1745
2.5332
2.9241
Elapsed time is 3.549135 seconds.
```

Physicists like to plot the eigenmodes shifted up by an amount equal to the eigenvalue:

```colors = [1 0 0; 0 .8 0; .9 .9 0; 0 0 1; 1 0 1; 0 .8 1];
for j = 1:neigs
v = EV(:,j)/15; d = D(j,j);
if max(v)<-min(v), v = -v; end
plot(d+v,LW,2,'color',colors(1+mod(j-1,6),:))
end
``` There is a great deal of such physics in such pictures. The lower eigenmodes correspond to particles trapped on one side or the other, with a state function decreasing exponentially within the barrier. At higher energies the particles are not localized.