Robert Bukowski

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.

Helium dimer potential from symmetry-adapted perturbation theory calculations using large Gaussian geminal and orbital basis sets

The symmetry-adapted perturbation theory (SAPT) has been employed to calculate an accurate potential energy curve for the helium dimer. For major components of the interaction energy, saturated values have been obtained using extended Gaussian-type geminal bases. Some other, less significant components were computed using a large orbital basis and the standard set of SAPT codes. The remaining small fraction of the interaction energy has been obtained using a nonstandard SAPT program specific for two-electron monomers and the supermolecular full configuration interaction (FCI) calculations in a moderately large orbital basis. Accuracy of the interaction energy components has been carefully examined. The most accurate to date values of the electrostatic, exchange, induction, and dispersion energies are reported for distances from 3.0 to 7.0 bohr. After adding the retardation correction predicted by the Casimir theory, our new potential has been shown [A. R. Janzen and R. A. Aziz (submitted)] to recover the ...

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...

Density-Fitting Method in Symmetry-Adapted Perturbation Theory Based on Kohn-Sham Description of Monomers.

With the DF-SAPT(DFT) approach, RDX dimer can be calculated within a reasonable CPU time. This system is beyond the reach of correlated ab initio methods such as CCSD(T). SAPT(DFT) also provides an insight into physical picture of the interactions by decomposing the interaction energy into physical contributions. The analysis of the interaction energy components suggests that the dispersion interaction is very important, at least for some of the geometrical configurations of the RDX dimer and, therefore, methods that are unable to properly account for the dispersion interaction (such as the supermolecular DFT method with standard density functionals) cannot be expected to yield correct results. With the full potential energy surface of the RDX dimer which is under development, we are able to model the properties of this important energetic material

Pair potential for water from symmetry-adapted perturbation theory

The interaction energies of over a thousand water dimer configurations have been calculated using the symmetry-adapted perturbation theory. Effective, interaction optimized bases were used leading to 0.2 kcal/mol accuracy near the minimum of the dimer potential. The computed points were then fitted to two types of analytic potential energy surfaces, a site-site form and an expansion in functions dependent on the vector connecting the centers of mass and on the Euler angles defining the orientation of each monomer. The second virial coefficient was calculated from these surfaces including the quantum correction and isotopic dependence, as well as the molar heat capacity at constant pressure. Comparison of these data to experiment shows that both of our surfaces are superior to any previously available.The interaction energies of over a thousand water dimer configurations have been calculated using the symmetry-adapted perturbation theory. Effective, interaction optimized bases were used leading to 0.2 kcal/mol accuracy near the minimum of the dimer potential. The computed points were then fitted to two types of analytic potential energy surfaces, a site-site form and an expansion in functions dependent on the vector connecting the centers of mass and on the Euler angles defining the orientation of each monomer. The second virial coefficient was calculated from these surfaces including the quantum correction and isotopic dependence, as well as the molar heat capacity at constant pressure. Comparison of these data to experiment shows that both of our surfaces are superior to any previously available.

Ab initio three-body interactions for water. I. Potential and structure of water trimer

A new ab initio three-body potential for water has been generated from the Hartree–Fock method and symmetry-adapted perturbation theory calculations performed at 7533 trimer geometries. The calculated nonadditive energies were then fitted to a physically motivated analytic formula containing representations of short-range exchange contributions and damped induction terms. To our knowledge, this is the first time the short-range nonadditive interactions have been explicitly included in a potential for water. The fitted nonadditive potential was then applied, together with an accurate ab initio pair potential, SAPT-5s, to evaluate the effects of nonadditivity on the structure and energetics of water trimer.

Water pair potential of near spectroscopic accuracy. II. Vibration–rotation–tunneling levels of the water dimer

Nearly exact six-dimensional quantum calculations of the vibration‐rotation‐tunneling ~VRT! levels of the water dimer for values of the rotational quantum numbers J and K <2 show that the SAPT-5s water pair potential presented in the preceding paper ~paper I! gives a good representation of the experimental high-resolution far-infrared spectrum of the water dimer. After analyzing the sensitivity of the transition frequencies with respect to the linear parameters in the potential we could further improve this potential by using only one of the experimentally determined tunneling splittings of the ground state in (H 2O) 2. The accuracy of the resulting water pair potential, SAPT-5st, is established by comparison with the spectroscopic data of both (H2O) 2 and (D2O) 2: ground and excited state tunneling splittings and rotational constants, as well as the frequencies of the intermolecular vibrations.

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.

On the role of bond functions in interaction energy calculations: Ar⋅⋅⋅HCl, Ar⋅⋅⋅H2O, (HF)2

We analyze the effect of an extended set of bond functions on the SCF and MP2 interaction energies, and their SAPT perturbation components; electrostatic, induction, dispersion, and exchange. The electrostatic, induction, and exchange terms at the SCF level prove to be largely independent. The dispersion energy is substantially improved and the improvement did not depend much on the bond‐function location. In contrast, the electrostatic‐correlation term is usually seriously distorted and the distortion strongly dependent on the bond‐function location. It was also shown that the distortion may be significantly reduced by appropriate shifting of the location. Only then the interaction energies obtained with bond functions may be considered reliable. It is strongly recommended to control the electrostatic‐correlation term. We also present samples of accurate results (within 5% error bar) for the Ar–HCl, Ar–H2O, and (HF)2 complexes.

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.