CI study of the water dimer potential surface

Many quantum mechanical calculations indicate water molecules in the gas and liquid phase have much larger quadrupole moments than any of the common site models of water for computer simulations. Here, comparisons of multipoles from quantum mechanical/molecular mechanical (QM/MM) calculations at the MP2/aug-cc-pVQZ level on a B3LYP/aug-cc-pVQZ level geometry of a waterlike cluster and from various site models show that the increased square planar quadrupole can be attributed to the p-orbital character perpendicular to the molecular plane of the highest occupied molecular orbital as well as a slight shift of negative charge toward the hydrogens. The common site models do not account for the p-orbital type electron density and fitting partial charges of TIP4P-or TIP5P-type models to the QM/MM dipole and quadrupole give unreasonable higher moments. Furthermore, six partial charge sites are necessary to account reasonably for the large quadrupole, and polarizable site models will not remedy the problem unless they account for the p-orbital in the gas phase since the QM calculations show it is present there too. On the other hand, multipole models by definition can use the correct multipoles and the electrostatic potential from the QM/MM multipoles is much closer than that from the site models to the potential from the QM/MM electron density. Finally, Monte Carlo simulations show that increasing the quadrupole in the soft-sticky dipolequadrupole-octupole multipole model gives radial distribution functions that are in good agreement with experiment.

Section: Introductionmentioning

confidence: 99%

The large quadrupole of water molecules

Niu

Tan

Ichiye

2011

“…39 Ab initio calculations on the ͓Cr͑H 2 O͒ 6 ͔ ϩ3 ͑H 2 O͒ 12 cluster showed that almost half of the overestimation ͑about 65 kcal/mol͒ is due to the many-body effects present in the second hydration shell. In the simulations performed for this cation, water-water interactions were described by nonpolarizable water models: MCY 48 and TIP4P. 49 In principle, a realistic potential should be able to accurately reproduce all the properties of the system being simulated; the validation against the available experimental data is a requirement to assess the reliability of the model in describing properties that have not been determined experimentally, and in providing an explanation in terms of intermolecular interactions.…”

Section: Introductionmentioning

confidence: 99%

Coupling a polarizable water model to the hydrated ion–water interaction potential: A test on the Cr3+ hydration

Martı́nez

Hernández-Cobos

Saint-Martı́n

et al. 2000

A strategy to build interaction potentials for describing ionic hydration of highly charged monoatomic cations by computer simulations, including the polarizable character of the solvent, is proposed. The method is based on the hydrated ion concept that has been previously tested for the case of Cr 3ϩ aqueous solutions ͓J. Phys. Chem. 100, 11748 ͑1996͔͒. In the present work, the interaction potential of ͓Cr͑H 2 O 6 ͔͒ 3ϩ with water has been adapted to a water model that accounts for the polarizable character of the solvent by means of a mobile charge harmonic oscillator representation ͑MCHO model͒ ͓J. Chem. Phys. 93, 6448 ͑1990͔͒. Monte Carlo simulations of the Cr 3ϩ hexahydrate plus 512 water molecules have been performed to study the energetics and structure of the ionic solution. The results show a significant improvement in the estimate of the hydration enthalpy ͓⌬H hydr ͑Cr 3ϩ ͒ϭϪ1109.6Ϯ70 kcal/mol͔ that now matches the experimental value within the uncertainty of this magnitude. The use of the polarizable water model lowers by ϳ140 kcal/mol the statistical estimation of the ͓Cr͑H 2 O 6 ͔͒ 3ϩ hydration enthalpy compared to the nonpolarizable model. ͑Ϫ573 kcal/mol for the polarizable model vs Ϫ714 kcal/mol for the nonpolarizable one.͒ This improvement reflects a more accurate treatment of the many-body nonadditive effects.

“…2 The MCY potential is a site-site form including Coulomb terms and double exponential terms fitted to 66 configuration interaction dimer energies. This form was quite successful at predicting observables despite the very small number of grid points and, from the current perspective, rather inaccurate values of ab initio computed energies.…”

Section: Introductionmentioning

confidence: 99%

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

Mas

Bukowski

Szalewicz

et al. 2000

170

196

A new ab initio pair potential for water was generated by fitting 2510 interaction energies computed by the use of symmetry-adapted perturbation theory ͑SAPT͒. 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 ͑1997͔͒. SAPT-5s has been shown to quantitatively predict the water dimer spectra, see the following paper ͑paper II͒. 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 5.50Ϯ0.01 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 estimatetogether with the dissociation energy D 0 computed from SAPT-5s-leads to a new prediction of the limit value of D 0 equal to 1165Ϯ54 cm Ϫ1 , close to but significantly more accurate than the best empirical value.