The Role of Correlation Effects in Non-Valence Anions

Non-valence anions are especially challenging for traditional electronic structure methods due to the need to use very large basis sets and to include high-order electron correlation effects. The most widely studied class of non-valence anions are dipole-bound anions for which the excess electron binds in the Hartree-Fock approximation provided a sufficiently flexible basis set is used. Even more challenging are non-valence correlation bound (NVCB) anions for which the excess electron does not bind in the absence of dispersion-like correlation effects between the excess electron and the electrons of the molecule or cluster. In my talk, ab initio and model Hamiltonian approaches for characterizing correlation effects in dipole-bound and NVCB anions will be analyzed. Although those anions have very extended charge distributions, we find that the dominant correlation contributions to the electron binding energies are shorter-range in nature than is generally believed.