Spectrum of one-dimensional complex-scaled single-particle
Hamiltonian with potential
$v(x) = 3(x^2 - 2) \ee^{-x^2 / 4}$ and $\theta=0.5$.
The lowest-energy resonance, here located close to the origin,
does not lie exactly on the real axis but has an
imaginary part of about $-10^{-5}$.