Complex eigenvalues, resonances, play an important role in large variety of
fields in physics and chemistry. For example, in cold molecular collision
experiments and electron scattering experiments, autoionizing and
pre-dissociative metastable resonances are generated. However, the computation
of complex resonance eigenvalues is difficult, since it requires severe
modifications of standard electronic structure codes and methods. Here we show
how resonance eigenvalues, positions and widths, can be calculated using the
standard, widely used, electronic-structure packages. Our method enables the
calculations of the complex resonance eigenvalues by using analytical
continuation procedures (such as Pad\'{e}). The key point in our approach is
the existence of narrow analytical passages from the real axis to the complex
energy plane. In fact, the existence of these analytical passages relies on
using finite basis sets. These passages become narrower as the basis set
becomes more complete, whereas in the exact limit, these passages to the
complex plane are closed.
As illustrative numerical examples we calculated the autoionization
resonances of helium, hydrogen anion and hydrogen molecule. We show that our
results are in an excellent agreement with the results obtained by other
theoretical methods and with available experimental results
