Accuracy and limitations of second-order many-body perturbation theory for predicting vertical detachment energies of solvated-electron clusters

John M. Herbert, Martin Head-Gordon
2006 Physical Chemistry, Chemical Physics - PCCP  
Vertical electron detachment energies (VDEs) are calculated for a variety of (H 2 O) n À and (HF) n À isomers, using different electronic structure methodologies but focusing in particular on a comparison between second-order Møller-Plesset perturbation theory (MP2) and coupled-cluster theory with noniterative triples, CCSD(T). For the surface-bound electrons that characterize small (H 2 O) n À clusters (n r 7), the correlation energy associated with the unpaired electron grows linearly as a
more » ... ction of the VDE but is unrelated to the number of monomers, n. In every example considered here, including strongly-bound "cavity" isomers of (H 2 O) 24 À , the correlation energy associated with the unpaired electron is significantly smaller than that associated with typical valence electrons. As a result, the error in the MP2 detachment energy, as a fraction of the CCSD(T) value, approaches a limit of about À7% for (H 2 O) n À clusters with VDEs larger than about 0.4 eV. CCSD(T) detachment energies are bounded from below by MP2 values and from above by VDEs calculated using second-order many-body perturbation theory with molecular orbitals obtained from density functional theory. For a variety of both strongly-and weaklybound isomers of (H 2 O) 20 À and (H 2 O) 24 À , including both surface states and cavity states, these bounds afford typical error bars of AE0.1 eV. We have found only one case where the Hartree-Fock and density functional orbitals differ qualitatively; in this case the aforementioned bounds lie 0.4 eV apart, and second-order perturbation theory may not be reliable. À
doi:10.1039/b513098k pmid:16482246 fatcat:nryizs4ptzbp3dwrdmdjmi2kl4