Model SCF calculations with a Rydberg-augmented basis set for CH3CN˙– in a ‘box’ of 22 helium atoms demonstrate that the cavity effect of condensed phases is to contract the singly occupied molecular orbital of weakly bound radical anions towards a valence-like orbital. This effect can be simulated by limiting the basis set to valence-like functions. The structure of the acetonitrile radical anion in the matrix and the activation energy for the combination of a methyl radical with a cyanide ion can be calculated with the diffuse-augmented 3–21 + G basis set. Similarly the 6–31 + G* basis set can be used to simulate the behaviour of the CH3Cl˙– radical anion in condensed phases. The 19 kcal mol–1 barrier to dissociation of the dipole-bound anion to CH˙3+ Cl– disappears in condensed phases, the potential curve being purely dissociative. The SiH3Cl˙– radical anion can also be treated well by valence-only basis sets, although there is probably no thermodynamically bound radical anion in the gas phase.