How to choose a one-electron basis set to reliably describe a dipole-boundanion

The problem of choosing appropriate atomic orbital basis sets for ab initio calculations on dipole-bound anions has been examined. Such basis sets are usually constructed as combination of a standard valence-type basis, desig ned to describe the neutral molecular core, and an extra diffuse set design ed to describe the charge distribution of the extra electron. As part of th e present work, it has been found that the most commonly used valence-type basis sets (e.g., 6-31++G** or 6-311+G**), when so augmented, are subject t o unpredictable under- or overestimating electron binding energies for dipo le-bound anions. Whereas, when the aug-cc-pVDZ, aug-cc-pVTZ [or other mediu m-size polarized (MSP) basis sets] are so augmented, more reliable binding energies are obtained especially when the electron binding energy is calcul ated at the CCSD(T) level of theory. The issue of designing and centering t he extra diffuse basis functions for the excess electron has also been stud ied, and our findings are discussed here. (C) 2000 John Wiley & Sons, Inc.