I wrote a column for the newsletter of our institute. Since I liked the result, I'm also reposting it below.
As a child I had a book "Bad advice" that contained nothing but poems suggesting you to do what you should really never do. So here is my bad professional advice (except that I won't risk making poetry):
Very often when we compute the spectrum of a Hamiltonian over a finite grid of parameter values, we cannot resolve whether crossings are avoided or not. Further if we only compute a part of the spectrum using e.g. a sparse diagonalization routine, we fail to find a proper sequence of levels.
Let us illustrate these two failure modes.
# Just some initialization
%matplotlib inline
import numpy as np
from scipy import linalg
from scipy.optimize import linear_sum_assignment
import matplotlib
from matplotlib import pyplot
matplotlib.rcParams['figure.figsize'] = (8, 6)
def ham(n):
"""A random matrix from a Gaussian Unitary Ensemble."""
h = np.random.randn(n, n) + 1j*np.random.randn(n, n)
h += h.T.conj()
return h
def bad_ham(x, alpha1=.2, alpha2=.0001, n=10, seed=0):
"""A messy Hamiltonian with a bunch of crossings."""
np.random.seed(seed)
h1, h2, h3 = ham(n), ham(n), ham(n)
a1, a2 = alpha1 * ham(2*n), alpha2 * ham(3*n) * (1 + 0.1*x)
a2[:2*n, :2*n] += a1
a2[:n, :n] += h1 * (1 - x)
a2[n:2*n, n:2*n] += h2 * x
a2[-n:, -n:] += h3 * (x - .5)
return a2
xvals = np.linspace(0, 1)
data = [linalg.eigvalsh(bad_ham(x)) for x in xvals]
pyplot.plot(data)
pyplot.ylim(-2.5, 2.5);
This is mock data produced by a random Hamiltonian with a bunch of crossings. We know that some of these apparent avoided crossings are too tiny to resolve, and should instead be classified as real crossings.
Let's now simulate what would happen if we also use sparse diagonalization to obtain some number of eigenvalues closest to 0.
truncated = [sorted(i[np.argsort(abs(i))[:13]]) for i in data]
pyplot.plot(truncated);
The ugly jumps are not real, they appear merely because some levels exit our window and new ones enter.
A desperate person who needs results right now at this point replots the data using a scatterplot.
pyplot.plot(truncated, '.');
This is OK, but at the points where the lines are dense our eye identifies vertical lines, making the plot harder to interpret.