  Hybrid quantum mechanics / molecular mechanics (QM/MM) models successfully
describe the properties of biological macromolecules. However, most QM/MM
methodologies are constrained to unrealistic gas phase models, thus limiting
their applicability. In the literature, several works have attempted to define
a QM/MM model in periodic boundary conditions (PBC) but frequently the models
are too time-consuming for general applicability to biological systems in
solution. Here, we define a simple and efficient electrostatic embedding QM/MM
model in PBC combining the benefits of electrostatic potential fitted (ESPF)
atomic charges and particle-mesh Ewald sums, that can efficiently treat systems
of arbitrary size at a reasonable computational cost. To illustrate this, we
apply our scheme to extract the lowest singlet excitation energies from a model
for arabidopsis thaliana cryptochrome 1 containing circa 93000 atoms,
reproducing accurately the experimental absorption maximum.
