Electron affinity of NH: a coupled-cluster and Hartree-Fock density-functional-theory study

We study the highly sensitive electron affinity of the NH radical at several levels of ab initio correlated theory (CCSD, CCSD + T(CCSD), CCSD(T), CCSDT, QCISD, QCISD(T)), and using Hartree-Fock density functional theory with different basis sets and reference functions. It is found that the contribution to the EA of NH from triple excitations exceeds the contributions from singles and doubles and accounts for the calculated EA value being in reasonable agreement with the experimental data (≈0.30 compared to 0.38 eV). The HFDFT/B3LYP approach was shown to fail in predicting the stability of the NH− anion with respect to autodetachment when the highest UHF occupied MO of NH− has a relatively large positive eigenvalue.