A force field for water has been developed entirely from first principles, without any fitting to experimental data. It contains both pairwise and many-body interactions. This force field predicts the properties of the water dimer and of liquid water in excellent agreement with experiments, a previously elusive objective. Precise knowledge of the intermolecular interactions in water will facilitate a better understanding of this ubiquitous substance.

Molecular scattering behavior has generally proven difficult to study at low collision energies. We formed a molecular beam of OH radicals with a narrow velocity distribution and a tunable absolute velocity by passing the beam through a Stark decelerator. The transition probabilities for inelastic scattering of the OH radicals with Xe atoms were measured as a function of the collision energy in the range of 50 to 400 wavenumbers, with an overall energy resolution of about 13 wavenumbers. The behavior of the cross-sections for inelastic scattering near the energetic thresholds was accurately measured, and excellent agreement was obtained with cross-sections derived from coupled-channel calculations on ab initio computed potential energy surfaces.

A new ab initio pair potential for water was generated by fitting 2510 interaction energies computed by the use of symmetry-adapted perturbation theory (SAPT). The new site–site functional form, named SAPT-5s, is simple enough to be applied in molecular simulations of condensed phases and at the same time reproduces the computed points with accuracy exceeding that of the elaborate SAPT-pp functional form used earlier [J. Chem. Phys. 107, 4207 (1997)]. SAPT-5s has been shown to quantitatively predict the water dimer spectra, see the following paper (paper II). It also gives the second virial coefficient in excellent agreement with experiment. Features of the water dimer potential energy surface have been analyzed using SAPT-5s. Average values of powers of the intermolecular separation—obtained from the ground-state rovibrational wave function computed in the SAPT-5s potential—have been combined with measured values to obtain a new empirical estimate of the equilibrium O–O separation equal to 5.50±0.01 bohr, significantly shorter than the previously accepted value. The residual errors in the SAPT-5s potential have been estimated by comparison to recent large-scale extrapolated ab initio calculations for water dimer. This estimate—together with the dissociation energy $D_0$ computed from SAPT-5s—leads to a new prediction of the limit value of $D_0$ equal to 1165±54 ${\mathrm{cm}}^{\text{−1}},$ close to but significantly more accurate than the best empirical value.

Watching collisions in the slow lane: Quantum mechanics aims to “micromanage” the details of collisions between atoms and molecules. However, it's hard to discern all the subtleties under high-energy conditions. Vogels et al. slowed down two intersecting beams of helium atoms and nitric oxide (NO) molecules to a relative crawl in order to characterize the collisions precisely. The data revealed short-lived resonances that matched theoretical predictions remarkably well—a striking feat on both sides, given the challenge of accurately modeling NO's unpaired electron. The study highlights chemists' increasingly sophisticated understanding of collision dynamics. Science , this issue p. 787

When Molecules Collide: As advances in computing power and algorithm design parallel the increasing sophistication of experimental apparatus, theory and measurement are perpetually trading places as to which can detail the dynamics of molecular interactions more precisely. At present, collisions of an atom with a diatomic molecule can be studied comparably in both domains. In contrast, collisions of two diatomics each bearing an unpaired electron manifest too many degrees of freedom for computational quantum mechanics. Kirste et al. (p. 1060 ) have now experimentally resolved the rotational dynamics of one such case—the inelastic scattering of NO + OH—and find that a simplified theoretical model focusing on long range interactions predicts the outcome surprisingly well. Such approximations could render many analogous systems moderately predictable.

Ultracold molecular collision dynamics: Ultracold collision dynamics are of great importance in understanding the quantum nature of chemical interactions, but achieving the ultracold regime for molecules is challenging. Traditional techniques based on alternative routes to assemble the ultracold atomic constituents are only able to produce a type of ultracold molecules that cannot probe state-selective dynamics. Using Stark deceleration and velocity map–imaging techniques, de Jongh et al. achieved the ultracold regime directly for a nitric oxide–helium system and measured the state-to-state cross sections for inelastic scattering with high precision (see the Perspective by Yang and Yang). The observed scattering resonances confirmed high sensitivity to the underlying interaction potential because only the most accurate electronic structure theory could reproduce their structure. Science , this issue p. 626 ; see also p. 582