Alessandro Tani

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...

Structure of a dilute aqueous solution of argon. A Monte Carlo simulation

A Monte Carlo simulation of a dilute aqueous solution of argon has been performed in the canonical (T,V,N) ensemble. The argon–water pair potential energy function has been obtained, with a best fitting procedure, from a set of 85 ab initio SCF energy values calculated with an extended basis set. Both energetic and structural evidences supporting the hypothesis of water structure promotion by nonpolar solutes are provided. Water molecules close to argon are oriented in a way similar to that of a crystalline clathrate cage. This analogy however is restricted to a local environment, which suggests that only parts of a real clathrate cage are present at a given time in this solution.

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...

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.

Diffusion effects of hydrogen bond fluctuations. I. The long time regime of the translational and rotational diffusion of water

A theoretical scheme to evaluate the long‐time effects of hydrogen bond fluctuations on water dynamics is carefully reexamined in the light of recent experimental results of quasielastic neutron scattering (QENS). A molecular dynamics computer simulation is also carried out on a fairly large (343 TIP4P molecules) sample and for a total time of ≈100 ps. at three temperatures. The length of the runs has allowed to obtain good results for a number of properties. Also the temperature dependence of the static dielectric constant agrees with the experimental data. Further, some crucial predictions of our theoretical model have been carefully checked. The prediction on the deviation from Fick’s law is found to be in a satisfactory accord with the results of our computer simulation. The reasons for the less satisfactory agreement with the low‐temperature results of QENS are discussed.

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.

The frequency and wavelength dependent dielectric permittivity of water

The static and dynamic dielectric behaviour of a model of liquid water is studied in the supercooled region, as a function of wavelength. The data used for this analysis were obtained in a previous molecular dynamics simulation of a sample of 343 water molecules, modelled by the TIP4P potential, at 245 K. A substantial wavelength dependence of the static dielectric permittivity is observed, while the value found for ϵ(0) seems to indicate that the TIP4P model is able to account qualitatively for the increase of dielectric constant upon temperature decrease, as found in real water. The time correlation function of the longitudinal and transverse components of the dipole density as well as that of the individual and total dipole moment is also calculated, to relate collective dielectric properties to the single molecule relaxation. The collective and single molecule dielectric relaxation times are obtained and their ratio seems to be close to a static property, namely the short-range orientational correlati...

A molecular dynamics study of the TIP4P model of water

Abstract Molecular dynamics calculations with a simple of 343 molecules are reported for the TIP4P model of water at the three temperatures −30.25 and 75°C. Structural and dynamical properties of this model are discussed in relation to experiment and other simulation results.

Collective motion and interparticle correlations in liquid water. A molecular dynamics simulation

The intramolecular and intermolecular contributions to the longitudinal current fluctuations of the hydrogens of liquid water have been computed by molecular dynamics simulation on the TIP4P model at 245 K. This indicates that the recently discovered high frequency (∼ 165 THz) ‘optical-like’ collective mode of the hydrogens is due to the intermolecular contributions and that these and the intramolecular part cancel each other in the frequency range 20–120 THz. Conversely, they mutually reinforce outside this region to produce the acoustical (∼8 THz) and ‘optical like’ collective bands, in the low-k region. The high frequency mode is rooted in the librational motion of the molecules. This is shown by a comparison of the intermolecular part spectrum with that of the interparticle correlation function of the projection of the angular velocity over the principal axes of inertia of the molecule. In particular, we are able to show that the high frequency band is almost entirely due to correlations of the rotati...

Solute-solute potential of mean force for non-polar molecules in water

A recently proposed extension of the Pratt-Chandler (PC) theory of the hydration of non-polar solutes has been applied to the calculation of solute-solute radial distribution functions, gaa (r)s. The difference between results obtained with the original and extended PC method is studied for solutes interacting with water through purely repulsive or full Lennard-Jones potentials. The role of the model of water on the calculated gaa (r) has been examined, for both the above interactions, comparing results computed with partial structure functions derived from computer simulation and neutron diffraction measurements. It is found that the model of water affects gaa (r) more than solute-water attractive forces. Finally, the size dependence of gaa (r) has been discussed with both PC methods and the results are qualitatively different for the largest solute considered, a Lennard-Jones model of neopentane. In particular, the contact configuration is excluded by the new approach and accepted by the original one.