Putative global minima for five intermolecular potential energy models are reported for water clusters (H2O)n with n ≤ 55. The models studied include three empirical, pairwise-additive potential energy surfaces, TIP4P, TIP4P-Ew, and TIP4P/2005, which use fixed point charges and rigid monomers. The other two, TTM2.1-F and AMOEBA, are polarizable, include non-additive inductive effects, have flexible monomers, and were parametrized, at least partially, using ab initio data. The n = 51 cluster has the same structure and is exceptionally stable for all five potentials. A structured inner core can be seen in cage clusters with n > 37. Periplanar rings, branched rings, and coils are among the structural motifs of the inner core.

Molecular interactions are partitioned in SCF and correlation energy parts. It is shown that for atomic systems, one can join high quality ’’a priori’’ SCF calculations with a semiempirical estimate of the correlation energy, made using the standard long range multipolar expansion and corrected assuming the 3Σ+u state of H2 as a model (scaling the ’’size’’ of the atomic charge distribution as the ionization potential to the negative 2/3 power) to obtain very good agreement with the available experimental information.

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Improved Roothaan–Hartree–Fock wave functions are reported for the ground states of all the neutral atoms from He to Xe, singly charged cations from Li+ to Cs+, and stable singly charged anions from H− to I−. Our neutral atom wave functions are an improvement over those of Clementi and Roetti [At. Data Nucl. Data Tables 14, 177 (1974)], Bunge et al. [Phys. Rev. A 46, 3691 (1992)] and Koga et al. [Phys. Rev. A 47, 4510 (1993)]. The ion wave functions are an improvement over those of Clementi and Roetti, and Koga et al. [J. Phys. B 26, 2529 (1993)]. In all cases, the current wave functions predict energies within 1.3×10−5 hartrees of the numerical Hartree–Fock limit.