Analysis of fourth-order Moller-Plesset limit energies: the importance of three-electron correlation effects

Fourth-order Moller-Plesset (MP4) correlation energies are computed for 28 atoms and simple molecules employing Dunning's correlation-consistent polar ized-valence m-zeta basis sets for m = 2, 3, 4, and 5. Extrapolation formul as are used to predict MP4 energies for infinitely large basis sets. It is shown that both total and partial MP4 correlation energies can be extrapola ted to limit values and that the sum of extrapolated partial MP4 energies e quals the extrapolated total MP4 correlation energy within calculational ac curacy. Therefore, partial MP4 correlation energies can be presented in the form of an MP4 spectrum reflecting the relative importance of different co rrelation effects. Typical trends in calculated correlation effects for a g iven class of electron systems are independent of the basis set used. As fi rst found by Cremer and He [(1996) J Phys Chem 100:6173], one can use MP4 s pectra to distinguish between electron systems with well-separated electron pairs and systems for which electrons cluster in a confined region of atom ic or molecular space. MP4 spectra for increasing size of the basis set rev eal that smaller basis set calculations underestimate the importance of thr ee-electron correlation effects for both classes by overestimating the impo rtance of pair correlation effects. The minimum size of a basis set require d for reliable MP4 calculations is given by a valence triple-zeta polarized basis, which even in the case of anions performs better than a valence dou ble-zeta basis augmented by diffuse functions.