A many-body version of the symmetry-adapted perturbation theory is developed for a direct calculation of intermolecular potentials as a sum of the electrostatic, exchange, induction, and dispersion contributions. Since no multipole expansion is used, the obtained interaction energy components are properly dampened at short distance by the charge-overlap (penetration) effects. The influence of the intramonomer correlation is accounted for by the perturbation expansion in terms of the Mo/ller–Plesset type fluctuation potentials WA and WB for the individual molecules. For the electrostatic and for the dispersion energy, the terms of the zeroth, first, and second order in WA+WB are considered. In this way, the leading three-particle correlation contribution to the dispersion energy is taken into account. As a test of our method, we have performed calculations of the interaction energy for the water and hydrogen fluoride dimers. Both the geometry and the basis set dependence of the interaction energy components have been investigated. For a comparison, we have also computed the supermolecular interaction energies through the full fourth order of the many-body perturbation theory. On the basis of our results, we predict the association energy for (H2O)2 equal to −4.7±0.2 kcal/mol in relatively poor agreement with the experimental value of −5.4±0.7 kcal/mol, but still within the experimental error bars. For (HF)2, the predicted association energy is −4.2±0.2 kcal/mol, while the experimental value (corrected by a theoretical zero-point energy) is −4.9±0.1 kcal/mol.

The second-order pair energies are calculated for the H2O molecule employing explicitly correlated Gaussian-type geminals. The selection of the basis functions is guided by the completeness criteria for Gaussian geminal bases of C2v symmetry and by the recently developed technique [J. Chem. Phys. 100, 1366 (1994)] of crude optimization of nonlinear geminal parameters. Several simplifications have been made in the matrix elements evaluations that substantially reduce the optimization time without sacrificing numerical accuracy. The present results provide the new best upper bound to the second-order correlation energy for the water molecule. These results are compared with those obtained by Kutzelnigg and co-workers using the basis set containing terms linear in the interelectronic distance.

The selected-ion flow tube (SIFT) technique has been employed in the study of reactions of carbon monoxide with the polyatomic carbon cations C+n(n=1−6) and CnH+(n=2–5) at 296±2 K in helium buffer gas at ∼0.34 Torr and 1.1×1016 atoms cm−3. The polyatomic carbon cations were generated by electron impact on a suitable parent molecule. Carbon monoxide was observed to add rapidly to C+n(n=2–6) twice in succession to form polyatomic carbon monoxide and dioxide cations, and once to CnH+ to form polyatomic carbon monoxide cations. Further additions did not occur with measurable specific rates. This remarkable behavior is attributed to double bond formation at the terminal carbon atoms of the polyatomic carbon cations. The specific rate for the addition of CO was observed to vary with the size of the polyatomic carbon cation, increasing to a maximum for reactions with five atoms in the reacting ion. This trend is attributed to an increase in the lifetime of the intermediate addition complex.

This paper describes a new method for synthesizing massive rare-earth hydrides with high hydrogen contents. The method can substantially reduce the time for synthesizing rare-earth hydrides compared to a conventional method. Thermally decomposed gas from a hydride was used to synthesize another hydride under very high pressure. AlH3 and MgH2 were used as hydrogen sources. When AlH3 was used as a solid source of hydrogen, SmH2.95 was obtained under the pressure of 4.0 GPa at 300 °C in 30 min. When MgH2 was used, SmH2.95 was obtained under 3.0 GPa at 250 °C for 1 h, or under 4.0 GPa at 200 °C for 1.5 h.