ABSTRACT: An infinitely diluted aqueous solution of Rb+ was studied using ab initio-based model potentials in classical Monte Carlo simulations to describe its structural and thermodynamic
features. An existing flexible and polarizable model [Saint-Martin et al. in J Chem Phys 113(24) 10899, 2000] was used for water–water interactions, and the parameters of the Rb+–water potential were fitted to reproduce the polarizability of the cation and a sample of ab initio pair interaction energies.
It was necessary to calibrate the basis set to be employed as a reference, which resulted in a new determination of the complete
basis set (CBS) limit energy of the optimal Rb+–OH2 configuration. Good agreement was found for the values produced by the model with ab initio calculations of three- and four-body
nonadditive contributions to the energy, as well as with ab initio and experimental data for the energies, the enthalpies
and the geometric parameters of Rb+(H2O)
clusters, with n=1, 2,…,8. Thus validated, the potential was used for simulations of the aqueous solution with three versions of the MCDHO
water model; this allowed to assess the relative importance of including flexibility and polarizability in the molecular model.
In agreement with experimental data, the Rb+–O radial distribution function (RDF) showed three maxima, and hence three hydration shells. The average coordination number
was found to be 6.9, with a broad distribution from 4 to 12. The dipole moment of the water molecules in the first hydration
shell was tilted to 55° with respect to the ion’s electric field and had a lower value than the average in bulk water; this
latter value was recovered at the second shell. The use of the nonpolarizable version of the MCDHO water model resulted in
an enhanced alignment to the ion’s electric field, not only in the first, but also in the second hydration shell. The hydration
enthalpy was determined from the numerical simulation, taking into account corrections to the interfacial potential and to
the spurious effects due to the periodicity imposed by the Ewald sums; the resulting value lied within the range of the various
different experimental data. An analysis of the interaction energies between the ion and the water molecules in the different
hydration shells and the bulk showed the same partition of the hydration enthalpy as for K+. The reason for this similarity is that at distances longer than 3Å, the ion–water interaction is dominated by the charge-(enhanced)
dipole term. Thus, it was concluded that starting at K+, the hydration properties of the heavier alkali metal cations should be very similar.
KeywordsRubidium ion hydration-Polarizable force fields-Monte Carlo simulation
Theoretical Chemistry Accounts 04/2012; 126(3):197-211. · 2.16 Impact Factor
ABSTRACT: A detailed study including ab initio calculations and classic Monte-Carlo simulations of hydroxylamine in the gas and liquid phases is presented. A classical interaction potential for hydroxylamine, which includes polarizability, many-body effects, and intramolecular relaxation, was constructed. The results of the simulation were compared to the available experimental data in order to validate the model. We conclude that liquid hydroxylamine has a multitude of hydrogen bonds leading to a large density where the existence of cis conformers and clusters of these conformers is possible. This explains the occurrence of the classical [R. Nast and I. Z. Foppl, Z. Anorg. Allg. Chem. 263, 310 (1950)] scheme for the molecule's decomposition at room temperature and its large exothermicity and instability.
The Journal of chemical physics 08/2011; 135(5):054502. · 3.09 Impact Factor
ABSTRACT: Monte Carlo simulations of liquid methanol were performed using a refined ab initio derived potential which includes polarizability, nonadditivity, and intramolecular relaxation. The results present good agreement between the energetic and structural properties predicted by the model and those predicted by ab initio calculations of methanol clusters and experimental values of gas and condensed phases. The molecular level picture of methanol shows the existence of both rings and linear polymers in the methanol liquid phase.
The Journal of Chemical Physics 12/2007; 127(22):224507-224507-14. · 3.33 Impact Factor
ABSTRACT: Four water models that have the same analytical potential but different degrees of freedom were used to examine the hydration
of Li+: (a) a polarizable and flexible molecule with constraints that account for the quantal nature of the vibration, (b) a polarizable
and classically flexible molecule, (c) a polarizable and rigid molecule, and finally (d) a nonpolarizable and rigid molecule.
The goal was to determine how individual molecular properties affect the correct description of the hydration of ions by comparing
the structural and thermodynamic predictions for the aqueous solution as made by the different models, which ranged from a
very refined one to a simple effective potential. The length of the Monte Carlo runs was large enough to ensure convergence
and provide statistically meaningful results; the four models attained good agreement with the experimental data available
for the hydration of Li+, as well as with the results of the most refined simulations. A well-defined first hydration shell was found. It had four
water molecules whose dipoles were not aligned to the electric field of the ion because of their hydrogen-bonding with water
molecules in outer shells. In the case of the most refined water model, the results showed this pattern clearly. On the other
hand, the rigid nonpolarizable version produced a slightly higher hydration number and an almost complete alignment of the
dipoles to the ion’s electric field. Moreover, a detailed analysis of a microscopic molecular model of hydration showed that
the average intramolecular geometry of the water molecules in the first hydration shell was the same as the one for those
in the bulk, whereas the electric field of the ion induced a dipole 0.2 D higher in the water molecules of the first hydration
shell. The value of the bulk was recovered at the second shell, which explains the good performance of the simplest model.
Thus, despite the differences found in the description of the first hydration shell between the polarizable and the nonpolarizable
models, the major effect on the polarization of the water molecules resulted from the water-water interaction.
Theoretical Chemistry Accounts 02/2006; 115(2):177-189. · 2.16 Impact Factor
ABSTRACT: Coexistence properties for water near the critical point using several ab initio models were calculated using grand canonical Monte Carlo simulations with multiple histogram reweighting techniques. These models, that have proved to yield a good reproduction of the water properties at ambient conditions, perform rather well, improving the performance of a previous ab initio model. It is also shown that bulk geometry and dipole values, predicted by the simulation, can be used and a good approximation obtained with a polarizable rigid water model but not when polarization is excluded.
The Journal of Chemical Physics 08/2005; 123(4):044506. · 3.33 Impact Factor
ABSTRACT: Up to now it has not been possible to neatly assess whether a deficient performance of a model is due to poor parametrization of the force field or the lack of inclusion of enough molecular properties. This work compares several molecular models in the framework of the same force field, which was designed to include many-body nonadditive effects: (a) a polarizable and flexible molecule with constraints that account for the quantal nature of the vibration [B. Hess, H. Saint-Martin, and H. J. C. Berendsen, J. Chem. Phys. 116, 9602 (2002), H. Saint-Martin, B. Hess, and H. J. C. Berendsen, J. Chem. Phys. 120, 11133 (2004)], (b) a polarizable and classically flexible molecule [H. Saint-Martin, J. Hernandez-Cobos, M. I. Bernal-Uruchurtu, I. Ortega-Blake, and H. J. C. Berendsen, J. Chem. Phys. 113, 10899 (2000)], (c) a polarizable and rigid molecule, and finally (d) a nonpolarizable and rigid molecule. The goal is to determine how significant the different molecular properties are. The results indicate that all factors--nonadditivity, polarizability, and intramolecular flexibility--are important. Still, approximations can be made in order to diminish the computational cost of the simulations with a small decrease in the accuracy of the predictions, provided that those approximations are counterbalanced by the proper inclusion of an effective molecular property, that is, an average molecular geometry or an average dipole. Hence instead of building an effective force field by parametrizing it in order to reproduce the properties of a specific phase, a building approach is proposed that is based on adequately restricting the molecular flexibility and/or polarizability of a model potential fitted to unimolecular properties, pair interactions, and many-body nonadditive contributions. In this manner, the same parental model can be used to simulate the same substance under a wide range of thermodynamic conditions. An additional advantage of this approach is that, as the force field improves by the quality of the molecular calculations, all levels of modeling can be improved.
The Journal of Chemical Physics 07/2005; 122(22):224509. · 3.33 Impact Factor
ABSTRACT: In this work we present a new proposal to model intermolecular interactions and use it for water molecules. The parameters of the model were fitted to reproduce the single molecule’s electrostatic properties, a sample of 352 points in a refined ab initio single molecule deformation potential energy surface (PES), and the theoretical limit of the dimerization energy, −20.8 kJ/mol. The model was able to reproduce a sample of 180 additional points in the single molecule deformation PES, and 736 points in a pair-interaction surface computed at the MP2/aug-cc-pVQZ′ level with the counterpoise correction. Though the model reproduced the diagonal of the polarizability tensor, it could account for only 60% of the three-body nonadditive contributions to the interaction energies in 174 trimers computed at the MP2/6-311++(2d,2p) level with full counterpoise correction, but reproduced the four-body nonadditivities in 34 tetramers computed at the same level as the trimers. The model’s predictions of the structures, energies, and dipoles of small clusters resulted in good agreement with experimental data and high quality ab initio calculations. The model also reproduced the second virial coefficient of steam at various temperatures, and the structure and thermodynamical properties of liquid water. We found that the short-range water–water interactions had a critical influence on the proper performance of the model. We also found that a model based on the proper intermolecular interactions requires the inclusion of intramolecular flexibility to be adequate. © 2000 American Institute of Physics.
The Journal of Chemical Physics. 12/2000; 113(24):10899-10912.
ABSTRACT: 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 Cr3+ aqueous solutions [J. Phys. Chem. 100, 11748 (1996)]. In the present work, the interaction potential of [Cr(H2O6)]3+ 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 model) [J. Chem. Phys. 93, 6448 (1990)]. Monte Carlo simulations of the Cr3+ 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 [ΔHhydr(Cr3+)=−1109.6±70 kcal/mol] that now matches the experimental value within the uncertainty of this magnitude. The use of the polarizable water model lowers by ∼ 140 kcal/mol the statistical estimation of the [Cr(H2O6)]3+ hydration enthalpy compared to the nonpolarizable model. (−573 kcal/mol for the polarizable model vs −714 kcal/mol for the nonpolarizable one.) This improvement reflects a more accurate treatment of the many-body nonadditive effects. © 2000 American Institute of Physics.
The Journal of Chemical Physics 01/2000; 112(5):2339-2347. · 3.33 Impact Factor
ABSTRACT: The many-body decomposition of the interaction energy for BeN and LiN (N=2 to 4) clusters is calculated in two approximations: the self-consistent-field method and the Mo/ller-Plesset perturbation theory up to the fourth order. This allows us to estimate the electron-correlation contributions to the many-body forces. The explicit expressions for these contributions in the perturbation theory formalism are obtained. We present a comparative analysis of the role of electron correlations in the BeN and LiN cluster formations and in the many-body interactions in these clusters. As follows from our results, the contribution of electron correlation to many-body interactions is essential for both the BeN and LiN clusters, especially for the latter ones, where nonadditivities are surprisingly large. © 1996 The American Physical Society.
Phys. Rev. A. 53(4).