Humberto Saint-Martin

A mobile charge densities in harmonic oscillators (MCDHO) molecular model for numerical simulations: The water–water interaction

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, en...

A comparative study of the hydration of Na+ and K+ with refined polarizable model potentials

The hydrations of Na+ and K+ were investigated by means of Monte Carlo simulations with refined ab initio based potentials. These interaction potentials include intramolecular relaxation, polarizability and many-body nonadditive effects. Care was taken to ensure proper convergence of the MC runs and that the statistical samples were large enough. As a result, agreement was attained with all experimental data available for the hydration of the ions. The water molecules in the first hydration shell were found to have the same intramolecular geometries and dipole moments as those of the bulk. Furthermore, their dipoles were not aligned to the electric field produced by the ion, but quite tilted. The hydration number for the sodium was found to be 5 or 6 water molecules, whereas the potassium’s hydration number had a probability distribution ranging from 5 to 10. From an analysis of the energetic contributions of each hydration shell to the total enthalpy of hydration we propose that the hydrated ions have a ...

Dynamics and Structure of Ln(III)―Aqua Ions: A Comparative Molecular Dynamics Study Using ab Initio Based Flexible and Polarizable Model Potentials

Aqueous solutions of a light (Nd3+), a middle (Gd3+), and a heavy (Yb3+) lanthanide ion were studied using ab initio based flexible and polarizable analytical potentials in classical molecular dynamics simulations to describe their thermodynamic, structural, and dynamic features. To avoid the spurious demise of O-H bonds, it was necessary to reparametrize an existing water model, which resulted in an improved description of pure water. The good agreement of the results from the simulations with the experimental hydration enthalpies, the Ln(III)-water radial distribution functions, and the water-exchange rates validated the potentials, though the r(Ln-Ow) distances were 6% longer than the experimentally determined values. A nona-coordinated state was found for Nd3+ in 95% of the simulation, with a tricapped trigonal prism (TCTP) geometry; the corresponding water-exchange mechanism was found to be of dissociative interchange (Id) character through a short-lived octa-coordinated transition state in a square antiprism (SQA) geometry. An octa-coordinated state in SQA geometry was found for Yb3+ in 99% of the simulation, and the observed exchange events exhibited characteristics of an interchange (I) mechanism. For Gd3+ an equilibrium was observed between 8-fold SQA and 9-fold TCTP coordinated states that was maintained by the frequent exchange of a water molecule from the first hydration shell with the bulk, thus producing significant deviations from the ideal geometries, and a fast exchange rate. Though strong water-water interactions prevented a full alignment of the dipoles to the ions electric field, the screening was found large enough as to limit its range to 5 A; water molecules further apart from the ion were found to have the same dipole as the molecules in the bulk, and a random orientation. The interplay among the water-ion and the water-water interactions determined the different coordination numbers and the different dynamics of the water exchange in the first hydration shell for each ion.

Nonadditivity in an analytical intermolecular potential: The water–water interaction

We present an analytical potential for intermolecular interactions which includes nonaddivity. We fitted this analytical form to an ab initio energy surface for the water–water interaction and looked into the fidelity of the fitting to two‐body interactions, as well as three‐body and four‐body nonadditives. We tested this potential in a Monte Carlo simulation of liquid water. The results obtained led us to conclude that the model performs very well, giving the best reported fit to the radial distribution functions, the enthalpy per water molecule, and other parameters. The potential appears as an inexpensive, accurate, and flexible potential to be used in numerical simulation studies.

Development of first-principles interaction model potentials. An application to the study of the bromide hydration

This work presents the development of first-principles bromide ion–water interaction potentials using the mobile charge density in harmonic oscillators-type model. This model allows for a flexible and polarizable character of the interacting molecules and has already been parametrized for water–water interactions. The prospected potential energy surfaces of the bromide ion-water system were computed quantum-mechanically at Hartree–Fock and Moller–Plesset second-order perturbation levels. In addition to the ion–solvent molecule pair, structures formed by the anion and two or three water molecules were considered in order to include many body effects. Minimizations of hydrated bromide clusters in gas phase [Br(H2O)n]− (n=1–6,10,15,20) and Monte Carlo computations of bromide aqueous solutions were performed to test the new potentials. Both structural and thermodynamic properties have been studied in detail and compared to the available experimental and theoretical values. From these comparisons, it was concl...

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

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±70u200akcal/mol] that now matches the experimental value within the uncertainty of this magnitude. The use of the polarizable wate...

Flexible constraints: An adiabatic treatment of quantum degrees of freedom, with application to the flexible and polarizable mobile charge densities in harmonic oscillators model for water

In classical molecular simulations chemical bonds and bond angles have been modeled either as rigid constraints, or as nearly harmonic oscillators. However, neither model is a good description of a chemical bond, which is a quantum oscillator that in most cases occupies the ground state only. A quantum oscillator in the ground state can be represented more faithfully by a flexible constraint. This means that the constraint length adapts itself, in time, to the environment, such that the rotational and potential forces on the constraint cancel out. An accurate algorithm for flexible constraints is presented in this work and applied to study liquid water with the flexible and the polarizable “mobile charge densities in harmonic oscillators” model. The iterations for the flexible constraints are done simultaneously with those for the electronic polarization, resulting in negligible additional computational costs. A comparison with fully flexible and rigidly constrained simulations shows little effect on structure and energetics of the liquid, while the dynamics is somewhat faster with flexible constraints.

Liquid methanol Monte Carlo simulations with a refined potential which includes polarizability, nonadditivity, and intramolecular relaxation

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.

Water liquid-vapor equilibria predicted by refined ab initio derived potentials.

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.

Ab initio calculations of the pyrophosphate hydrolysis reaction.

Ab initio quantum mechanical calculations were used to study the hydrolysis reaction H4P2O7 + H2O in equilibrium with 2H3PO4, as well as some molecular properties of the reactants and products. SCF calculations with several basis sets ranging from minimal to extended with polarization functions were used to look at the basis dependency of the reaction enthalpies and optimized geometries. Although the minimal basis sets yield erratic predictions of the enthalpy, when a more extended basis (3-21G*) was used for the geometry optimization, and the total energies of the reactants and products were computed with this and larger basis sets, we obtained more consistent predictions of the structural properties of the P-O-P bridge and of the heat of the hydrolysis reaction (delta E = -7.39 kcal/mol at the SCF/6-31G** level). A comparison is made with previous estimates performed with smaller basis sets and without taking into account the electron correlation effects, which are calculated in the present work. The inclusion of the zero point energy calculated using the harmonic approximation, and of the electronic correlation energy determined at the MBPT(2) level, raised the computed heat of the reaction to -3.83 kcal/mol, and when an estimate for the thermal energy was added, the value obtained was of -3.38 kcal/mol. In conclusion, we found that the hydrolysis of pyrophosphate should be exothermic in the gas phase. The implications of this result in relation to some recent theories about enzyme catalysis are discussed.