  Many-body Green's functions theory within the GW approximation and the
Bethe-Salpeter Equation (BSE) is implemented in the open-source VOTCA-XTP
software, aiming at the calculation of electronically excited states in complex
molecular environments. Based on Gaussian-type atomic orbitals and making use
of resolution of identify techniques, the code is designed specifically for
non-periodic systems. Application to the small molecule reference set
successfully validates the methodology and its implementation for a variety of
excitation types covering an energy range from 2-8 eV in single molecules.
Further, embedding each GW-BSE calculation into an atomistically resolved
surrounding, typically obtained from Molecular Dynamics, accounts for effects
originating from local fields and polarization. Using aqueous DNA as a
prototypical system, different levels of electrostatic coupling between the
regions in this GW-BSE/MM setup are demonstrated. Particular attention is paid
to charge-transfer (CT) excitations in adenine base pairs. It is found that
their energy is extremely sensitive to the specific environment and to
polarization effects. The calculated redshift of the CT excitation energy
compared to a nucelobase dimer treated in vacuum is of the order of 1 eV, which
matches expectations from experimental data. Predicted lowest CT energies are
below that of a single nucleobase excitation, indicating the possibility of an
initial (fast) decay of such an UV excited state into a bi-nucleobase CT
exciton. The results show that VOTCA-XTP's GW-BSE/MM is a powerful tool to
study a wide range of types of electronic excitations in complex molecular
environments.
