  Quantum chemistry simulations offer a cost-effective way for computational
design of BODIPY photosensitizers with potential use in photodynamic therapy
(PDT). However, accurate predictions of photophysical properties, such as
excitation energies, pose a challenge for the popular time-dependent density
functional theory (TDDFT) and equation-of-motion coupled cluster with singles
and doubles (EOM-CCSD) methods. By contrast, reliable descriptions can be
achieved by multi-reference quantum chemistry methods, though unfortunately,
their computational cost grows exponentially with the number of correlated
electrons. Alternatively, quantum computing holds a great potential for exact
simulation of photophysical properties in a computationally more efficient way.
To this end, we introduce the state-specific $\Delta$UCCSD-VQE (unitary coupled
cluster with singles and doubles variational quantum eigensolver) and
$\Delta$ADAPT-VQE methods in which the electronically excited state is
calculated via a non-Aufbau electronic configuration. The accuracy and
capability of the developed methods are assessed against experimentally
determined excitation energies for six BODIPY-derivatives. We show that the
proposed methods predict accurate vertical excitation energies that are not
only in good agreement with experimental reference data but also outperform
popular quantum chemistry methods, such as TDDFT and EOM-CCSD. Spurred by its
impressive performance and simplicity, we are confident that $\Delta$ADAPT will
emerge as the method of choice for guiding the rational design of
photosensitizers for PDT and photocatalysis in the era of near-term quantum
computing.
