Franca Maria Floris

A study of aqueous solutions of lanthanide ions by molecular dynamics simulation with ab initio effective pair potentials

Infinitely dilute aqueous solutions of Nd3+, Gd3+, and Yb3+ have been studied with molecular dynamics simulations, using ab initio effective ion–water pair potentials based on the polarizable continuum model. Structural results, as first peak positions of g(r) are in good agreement with experimental data. We obtain a coordination number of 9 for all the cations. This value agrees with experimental measurements for Nd3+ and Gd3+ but overestimates them for Yb3+. Significant differences between Yb3+ and the other two ions have however been observed in a detailed analysis of the solvation shell structure, based on the diagonalization of the inertia tensor. A similarity index based on a linear combination of the inertia tensor eigenvalues is proposed. Going from Nd3+ to Yb3+, structures like a capped square antiprism (CSQA) are more favored than a tricapped trigonal prism. In contrast to the lanthanide contraction observed for the most probable ion–oxygen distances, in CSQA structures the distance from the cen...

Free energy and entropy for inserting cavities in water: Comparison of Monte Carlo simulation and scaled particle theory results

The process of inserting cavities in water is studied with the aim of a better description of some of the terms necessary in continuum quantum mechanical models. Free-energy changes for the formation of soft and hard spherical cavities in TIP4P water have been computed by Monte Carlo (MC) simulation with statistical perturbation theory, up to a radius of 6 A. The free-energy change for the formation of a hard sphere, ΔGcav, is obtained combining the ΔGsol of a soft repulsive sphere with the ΔG corresponding to the process of transforming the soft sphere into a hard one. Two definitions of hard-sphere repulsive potentials have been considered, one only based on the distance of oxygens from the center of the cavity, while the other also excludes hydrogens from the same region. Differences in free energies are significant. The cubic polynomial expression ΔGcav, obtained by extrapolating the exact scaled particle theory (SPT) expression for very small excluding cavities, gives results in agreement with MC, wi...

Ab initio effective pair potentials for simulations of the liquid state, based on the polarizable continuum model of the solvent

Abstract We propose a method for determining effective pair potentials, incorporating n -body effects implicitly. The aim is to overcome the problem of non-additivity of solute—solute or solute—solvent interactions, so that accurate simulations of the liquid state can be made, without handling explicitly three-body or higher-order terms. The interaction potential between two charged or neutral species is evaluated from the wavefunction of the supermolecule, perturbed by the presence of the surrounding solvent. The solvent effect is simulated by means of the polarizable continuum model. The first application is a molecular dynamics study of the Fe 2+ and Fe 3+ cations in water. Using our effective potential we find the correct hydration number n h = 6, while a pair potential fitting in vacuo calculation yields n h = 8.

Hydration shell structure of the calcium ion from simulations with ab initio effective pair potentials

Abstract Computer simulations of dilute aqueous solutions of Ca 2+ have been performed with a strictly two-body ab initio ion-water potential and an effective ab initio pair potential. The latter includes many-body effects in an average way relying on a continuum polarizable model for the solvent. With the two-body potential, the structure of the first hydration shell of Ca 2+ turns out to be composed of nine molecules in an arrangement of approximate symmetry D 3h (hydration number=9.1). This structure is also observed with the effective potential, but it alternates with another that consists of eight molecules in an antiprism configuration, so that the average hydration number is 8.6.

Free energies and structures of hydrated cations, based on effective pair potentials

Abstract We present a method, based on a continuum representation of the solvent, to compute ab initio effective interaction potentials for solvated pairs. Such potentials take into account many-body effects, thus overcoming the non-additivity errors affecting uncorrected pair potentials. We apply the method to cation-water interactions, for a variety of cations: Li + , Be 2+ , Mg 2+ , Ca 2+ , Ni 2+ , Zn 2+ and Al 3+ . The potentials thus obtained are suitable for simulations of ionic solutions or clusters of water molecules surrounding a cation. We exploit them to compute hydration free energies ΔG hyd of cations, with the constraint that the first solvation shell contains a given number of water molecules. This enables us to find the thermodynamically most stable solvation number. The effective potential results compare well with experimental values of ΔG hyd and with full ab initio calculations on the [M(H 2 O) n ] q + complexes.

A computational ab-initio model for the evaluation of thermodynamic functions for solvent transfer processes.

Abstract A procedure addressed at the evaluation of solvent transfer free energy and then of partition coefficients, based on ab initio quantum mechanical procedures is here presented. Other thermodynamical transfer functions may be easily derived within this approach. There are no limitations, in principle, on the chemical composition of the solute and of the solvents, as well as on the temperature. The chemical composition of the material system may suggest however some simplifications, varying from case to case. The potentialities of the model are here illustrated with calculations concerning the water→ n-octanol transfer for hydrocarbons.

Evaluation of dispersion—repulsion contributions to the solvation energy. Calibration of the uniform approximation with the aid of RISM calculations

Abstract The dispersion—repulsion contributions to the solvation energy, computed with surface integrals and uniform approximation for some hydrocarbons in water and methanol, are compared with the results obtained using more realistic solvent distribution functions from RISM integral equation. The cavity surface we use is not spherical and models the structure of the enclosed solute molecule through a set of interlocking spheres centered on the interaction sites. The RISM radial distribution functions are used to fix the value of the radii of closest approach between solute—solvent interaction centers that define a minimal cavity surface. Two ways of working with this approximation are examined comparing also local contributions on the cavity surface. It is possible to find a set of cavity radii, to be used in the uniform approximation, that lead to dispersion—repulsion contributions in good agreement with those derived from RISM radial distribution functions and show a satisfactory degree of transferability.

Quantum Monte Carlo formulation of volume polarization in dielectric continuum theory

We present a novel formulation based on quantum Monte Carlo techniques for the treatment of volume polarization due to quantum mechanical penetration of the solute charge density in the solvent domain. The method allows to accurately solve Poissons equation of the solvation model coupled with the Schrodinger equation for the solute. We demonstrate the performance of the approach on a representative set of solutes in water solvent and give a detailed analysis of the dependence of the volume polarization on the solute cavity and the treatment of electron correlation.

Preferential solvation of Ca2+ in aqueous solutions containing ammonia: A molecular dynamics study

Ca2+ aqueous solutions containing different proportions of ammonia have been studied by means of molecular dynamics simulations. Previously developed ab initio effective pair potentials, in the framework of the polarizable continuum model, and only tested at a cluster computation level, have been employed to describe ion–ligand interactions. Structural and dynamic changes present in the neighborhood of the ion as a function of the ammonia concentration have been followed. Results show a preferential solvation for ammonia, even at very low concentrations. For the pure aqueous solution, calcium ion is coordinated by eight water molecules, while the presence of ammonia favors an equilibrium between an octa and enna-coordinated situation when this ligand becomes predominant, confirming the prediction of cluster calculations. However, the increase in the coordination number is followed by an intrinsic loss of stability for the identifiable solvated structures because of the larger tendency of ammonia to partic...

A density functional and quantum Monte Carlo study of glutamic acid in vacuo and in a dielectric continuum medium.

We present density functional theory (DFT) and quantum Monte Carlo (QMC) calculations of the glutamic acid and glutamate ion in vacuo and in various dielectric continuum media within the polarizable continuum model (PCM). In DFT, we employ the integral equation formalism variant of PCM while, in QMC, we use a PCM scheme we have developed to include both surface and volume polarization. We investigate the gas-phase protonation thermochemistry of the glutamic acid using a large set of structural conformations, and find that QMC is in excellent agreement with the best available theoretical and experimental results. For the solvated glutamic acid and glutamate ion, we perform DFT calculations for dielectric constants, ε, between 4 and 78. We find that the glutamate ion in the zwitterionic form is more stable than the non-zwitterionic form over the whole range of dielectric constants, while the glutamic acid is more stable in its non-zwitterionic form at ε = 4. The dielectric constant at which the two glutamic acid species have the same energy depends on the cavity size and lies between 5 and 12.5. We validate these results with QMC for the two limiting values of the dielectric constant, and find qualitative agreement with DFT even though the solvent polarization is less pronounced at the QMC level.