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.