Electronic Excitations of Polythiophene within Many-Body Perturbation Theory with and without the Tamm–Dancoff Approximation

Excited
electronic states of small and large π-conjugated
organic molecules can be described within ab initio many-body perturbation
theory, notably by the GW approximation for the electron self-energy
operator combined with the Bethe–Salpeter equation for correlated
electron–hole excitations.
In this context, the Tamm–Dancoff approximation is routinely
employed to reduce the computational effort. It is known that the
use of this approximation introduces errors of several 100 meV for
small organic molecules but is negligible for extended systems. In
this paper, we investigate how exactly the transition between these
two regimes happens, by calculating the optical excitation energies
of a series of polythiophene molecules of different sizes. We determine
which parts of the electron–hole interaction are responsible
for the deviation and show that the quantitative effects of the Tamm–Dancoff
approximation depend sensitively on the size of the electronic system,
in particular on the electronic conjugation length.