Ad van der Avoird

Predictions of the Properties of Water from First Principles

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.

N2-N2 interaction potential from ab initio calculations with application to the structure of (N2)2

The short range electrostatic and (first order) exchange contributions to the N2–N2 interaction energy have been calculated ab initio as a function of the N2 orientations and the distance (139 geometries). Using a numerical integration procedure, the results have been represented analytically in the form of a spherical expansion. At R=0.3 nm this expansion is accurate to better than 0.5% if we include the first 18 independent terms, to 2% if we truncate after LA=LB=4, and to 16% if we truncate after LA=LB=2. In combination with the long range multipole expansion results (electrostatic R−5, R−7, R−9 terms, dispersion R−6, R−8, R−10 terms) calculated by Mulder et al., this yields an anisotropic N2–N2 interaction potential in the region of the van der Waals minimum, which can be fairly well represented also by a site–site model. The potential is in good agreement with the available experimental data for the gas phase and for the ordered (α and γ) crystal phases of solid N2. The structure of the van der Waals...

Water pair potential of near spectroscopic accuracy. I. Analysis of potential surface and virial coefficients

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...

Quantum dynamics of the van der Waals molecule (N2)2: An ab initio treatment

Starting with an available ab initio N2–N2 potential, which favors a crossed equilibrium structure for the (N2)2 dimer with well depth De=122 cm−1, Re=3.46 A, and barriers to internal rotations of 25 and 40 cm−1, we calculate the bound rovibrational states of this dimer for J=0, 1, and 2. This is done by solving a secular problem over the exact (rigid monomer) Hamiltonian including centrifugal distortions and Coriolis interactions, using a product basis of radial (Morse oscillator) functions and angular momentum eigenfunctions. The full permutation‐inversion symmetry of the system, in relation to the nuclear spin coupling, is used in order to simplify the calculations and to derive selection rules for IR absorption. We find that the (N2)2 dimer has a large number of bound rovibrational states (92 already for J=0). These are analyzed by correlation with rigid molecule (harmonic oscillator/rigid rotor) results, on the one hand, and with the states of two freely rotating N2 monomers, on the other, and by plo...

A new He–CO interaction energy surface with vibrational coordinate dependence. I. Ab initio potential and infrared spectrum

The intermolecular potential energy surface of the He–CO complex including the CO bond length dependence has been calculated using symmetry-adapted perturbation theory (SAPT). The potential has a minimum of em=−23.734 cm−1 with Rm=6.53 bohr at a skew geometry (ϑm=48.4°) if the molecular bond length is fixed at the equilibrium value of 2.132 bohr. We have applied the potential in the calculation of bound state levels and the infrared spectrum for the 3He–CO and 4He–CO complexes. The computed ab initio transition frequencies are found to agree within 0.1 cm−1 with experiment. In paper II [J. P. Reid, H. M. Quiney, and C. J. S. M. Simpson, J. Chem. Phys. 107, 9929 (1997)], the potential surface is used to calculate vibrational relaxation cross sections and rate constants.

Polarizable interaction potential for water from coupled cluster calculations. II. Applications to dimer spectra, virial coefficients, and simulations of liquid water

The six-dimensional CC-pol interaction potential for the water dimer was used to predict properties of the dimer and of liquid water, in the latter case after being supplemented by a nonadditive potential. All the results were obtained purely from first principles, i.e., without any fitting to experimental data. Calculations of the vibration-rotation-tunneling levels of (H(2)O)(2) and (D(2)O)(2), a very sensitive test of the potential surface, gave results in good agreement with experimental high-resolution spectra. Also the virial coefficients and properties of liquid water agree well with measured values. The present model performs better than published force fields for water in a simultaneous reproduction of experimental data for dimer spectra, virials, and properties of the liquid.

New ab initio potential energy surface and the vibration-rotation-tunneling levels of (H2O)2 and (D2O)2

We report a new full-dimensional potential energy surface (PES) for the water dimer, based on fitting energies at roughly 30,000 configurations obtained with the coupled-cluster single and double, and perturbative treatment of triple excitations method using an augmented, correlation consistent, polarized triple zeta basis set. A global dipole moment surface based on Moller-Plesset perturbation theory results at these configurations is also reported. The PES is used in rigorous quantum calculations of intermolecular vibrational frequencies, tunneling splittings, and rotational constants for (H2O)2 and (D2O)2, using the rigid monomer approximation. Agreement with experiment is excellent and is at the highest level reported to date. The validity of this approximation is examined by comparing tunneling barriers within that model with those from fully relaxed calculations.

Quantum-state resolved bimolecular collisions of velocity-controlled oh with no radicals

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. Precise experiments on bimolecular collisions show that simplifications rendering theory tractable confer reasonable accuracy. Whereas atom-molecule collisions have been studied with complete quantum-state resolution, interactions between two state-selected molecules have proven much harder to probe. Here, we report the measurement of state-resolved inelastic scattering cross sections for collisions between two open-shell molecules that are both prepared in a single quantum state. Stark-decelerated hydroxyl (OH) radicals were scattered with hexapole-focused nitric oxide (NO) radicals in a crossed-beam configuration. Rotationally and spin-orbit inelastic scattering cross sections were measured on an absolute scale for collision energies between 70 and 300 cm−1. These cross sections show fair agreement with quantum coupled-channels calculations using a set of coupled model potential energy surfaces based on ab initio calculations for the long-range nonadiabatic interactions and a simplistic short-range interaction. This comparison reveals the crucial role of electrostatic forces in complex molecular collision processes.

Polarizable interaction potential for water from coupled cluster calculations. I. Analysis of dimer potential energy surface

A six-dimensional interaction potential for the water dimer has been fitted to ab initio interaction energies computed at 2510 dimer configurations. These energies were obtained by combining the supermolecular second-order energies extrapolated to the complete basis set limit from up to quadruple-zeta quality basis sets with the contribution from the coupled-cluster method including single, double, and noniterative triple excitations computed in a triple-zeta quality basis set. All basis sets were augmented by diffuse functions and supplemented by midbond functions. The energies have been fitted using an analytic form with the induction component represented by a polarizable term, making the potential directly transferable to clusters and the bulk phase. Geometries and energies of stationary points on the potential surface agree well with the results of high-level ab initio geometry optimizations.

Ab initio valence-bond calculations of the Van der Waals interactions between pi systems : the ethylene dimer

A multistructure valence‐bond method for the calculation of van der Waals forces is presented which includes in one consistent formalism the electrostatic, induction, and dispersion forces and takes exchange correctly into account. The application of this method to the ethylene dimer leads to the following main conclusions: (1) The ’’first order’’ electrostatic forces are comparable in magnitude to the ’’second order’’ forces even though the molecules possess no permanent dipole moments. Dispersion forces are much larger than induction. Second order interactions are more isotropic than first order forces. (2) In the multipole expansions of the long range forces, the inclusion of the first term only is not sufficient for a good approximation to the interaction. (3) Exchange effects become nonnegligible at approximately 12 bohr, while the van der Waals minimum between two perpendicular molecules is at 9.4 bohr. At about 6 bohr, penetration effects make the multipole expansion meaningless. Possible simplific...