  The benzene radical anion, well-known in organic chemistry as the first
intermediate in the Birch reduction of benzene in liquid ammonia, exhibits
intriguing properties from the point of view of quantum chemistry. Notably, it
has the character of a metastable shape resonance in the gas phase, while
measurements in solution find it to be experimentally detectable and stable. In
this light, our previous calculations performed in bulk liquid ammonia
explicitly reveal that solvation leads to stabilization. Here, we focus on the
transition of the benzene radical anion from an unstable gas-phase ion to a
fully solvated bound species by explicit ionization calculations of the radical
anion solvated in molecular clusters of increasing size. The computational cost
of the largest systems is mitigated by combining density functional theory with
auxiliary methods including effective fragment potentials or approximating the
bulk by polarizable continuum models. Using this methodology, we obtain the
cluster size dependence of the vertical binding energy of the benzene radical
anion converging to the value of $-$2.3 eV at a modest computational cost.
