J. Hernández-Rojas

Global minima for rare gas clusters containing one alkali metal ion

We present candidate structures for the global minima of N-atom rare gas clusters containing one additional alkali metal ion, LJNM. Lennard-Jones and Mason–Schamp potentials are used to represent the rare gas–rare gas and rare gas–alkali metal ion interactions, respectively. Results are presented for parameters appropriate to both Ar–K+ and Xe–Cs+ systems. When the ion is closer in size to the rare gas atoms (for XeNCs+) the global minima tend to be based on icosahedral packing. However, when the ion is relatively small (for ArNK+) the global minima below a certain size threshold are based on structures where the ion has lower coordination numbers. For larger clusters the global minima are again based on icosahedral packing. The latter structures can be found with minimal computational effort using the known global minima for clusters bound by Lennard-Jones or Morse potentials, substituting one atom at a time by the ion and minimizing.

Modeling water clusters on cationic carbonaceous seeds.

The Dang-Chang many-body polarizable potential has been used to model the interaction between water molecules and a cationic carbonaceous molecule X(+), with X = C(60) (buckminsterfullerene), C(24)H(12) (coronene), or C(20)H(10) (corannulene). The most stable structures of (H(2)O)(n)X(+), located with the basin-hopping method, consist of a water cluster next to the carbon cation but often deviate from those obtained for pure water clusters. The accuracy of the intermolecular potential is checked by performing dedicated high-level electronic structure calculations using the B97-1 density functional. Finally, some thermodynamical and dynamical manifestations of the nonwetting behavior are discussed.

Microcanonical versus canonical analysis of protein folding.

The microcanonical analysis is shown to be a powerful tool to characterize the protein folding transition and to neatly distinguish between good and bad folders. An off-lattice model with parameter chosen to represent polymers of these two types is used to illustrate this approach. Both canonical and microcanonical ensembles are employed. The required calculations were performed using parallel tempering Monte Carlo simulations. The most revealing features of the folding transition are related to its first-order-like character, namely, the S-bend pattern in the caloric curve, which gives rise to negative microcanonical specific heats, and the bimodality of the energy distribution function at the transition temperatures. Models for a good folder are shown to be quite robust against perturbations in the interaction potential parameters.

Lowest-energy structures of (C60)nX (X=Li+,Na+,K+,Cl-) and (C60)nYCl (Y=Li,Na,K) clusters for n</=13.

Basin-hopping global optimization is used to find likely candidates for the lowest minima on the potential energy surface of (C(60))(n)X (X=Li(+),Na(+),K(+),Cl(-)) and (C(60))(n)YCl (Y=Li,Na,K) clusters with n</=13. The energy is evaluated using the Girifalco form for the C(60) intermolecular potential along with a polarization potential, which depends on the first few nonvanishing C(60) multipole polarizabilities. We find that the ions occupy interstitial sites of a (C(60))(n) cluster, the coordination shell being triangular for Li(+), tetrahedral for Na(+) and K(+), and octahedral for Cl(-). When the required coordination site does not exist in the corresponding (C(60))(n) global minimum, the lowest minimum of the doped system may be based on an alternative geometry. This situation is particularly common in the Cl(-) complexes, where the (C(60))(n) global minima with icosahedral packing change into decahedral or closed-packed forms for the ions. In all the ions we find a significant binding energy for the doped cluster. In the alkali chloride complexes the preferred coordination for the diatomic moiety is octahedral and is basically determined by the Cl(-) ion. However, the smaller polarization energies in this case mean that a change in structure from the (C(60))(n) global minimum does not necessarily occur if there is no octahedral site.

Rotational spectra for off‐center endohedral atoms at C60 fullerene

Rotational spectra for endohedral Li+@C60 and Na+@C60 are calculated at different temperatures. Most of the features in these spectra are related with the degree of anisotropy in the atom–cage interaction. While the low anisotropy for Na+@C60 results in rather simple spectra with the 2B oscillation typical of a diatomic molecule, the more eccentric and anisotropic Li+@C60 produces complex spectra with rotational and librational bands. Some interesting effects are induced by the cage rotation, which has been incorporated through a semiclassical formalism.

Polarization effects in C60 fullerene complexes of alkali ions

We introduce a secular semiempirical model of the Pariser–Parr–Pople type to reproduce the electronic structure and polarizability of the C60 fullerene. The model is then used to simulate the response of this molecule to an electric charge and estimate its polarization energy. By expressing the charge potential at the C60-cage surface as a multipole expansion, an analytical form is obtained for the polarization energy. Application of these results to endo- and exohedral complexes of alkali ions gives data in rather good agreement with recent ab initio calculations [Hira and Ray, Phys. Rev. A 52, 141 (1995)].

Optimal covering of C60 fullerene by rare gases

Putative global energy minima of clusters formed by the adsorption of rare gases on a C(60) fullerene molecule, C(60)X(N) (X=Ne, Ar, Kr, Xe; N ≤ 70), are found using basin-hopping global optimization in an empirical potential energy surface. The association energies per rare gas atom as a function of N present two noticeable minima for Ne and Ar and just one for Kr and Xe. The minimum with the smallest N is the deepest one and corresponds to an optimal packing monolayer structure; the other one gives a monolayer with maximum packing. For Kr and Xe, optimal and maximum packing structures coincide. By using an isotropic average form of the X-C(60) interaction, we have established the relevance of the C(60) surface corrugation on the cluster structures. Quantum effects are relevant for Ne clusters. The adsorption of these rare gases on C(60) follows patterns that differ significantly from the ones found recently for He by means of experimental and theoretical methods.

Physical properties of small water clusters in low and moderate electric fields

Likely candidates for the lowest minima of water clusters (H(2)O)(N) for N ≤ 20 interacting with a uniform electric field strength in the range E ≤ 0.6 V/Å have been identified using basin-hopping global optimization. Two water-water model potentials were considered, namely TIP4P and the polarizable Dang-Chang potential. The two models produce some consistent results but also exhibit significant differences. The cluster internal energy and dipole moment indicate two varieties of topological transition in the structure of the global minimum as the field strength is increased. The first takes place at low field strengths (0.1 V/Å<E < 0.2 V/Å) and reorganizes the hydrogen-bonds to orient the water permanent dipoles along the field. The second type of transition occurs at larger field strengths (0.3 V/Å<E < 0.5 V/Å) and corresponds to an extensive structural reorganization, where several hydrogen-bonds break as the cluster stretches along the field direction, the larger clusters (N > 10) usually forming helical structures.

On polarization effects in endohedral fullerene complexes

Abstract Recently proposed semiempirical analytical forms for the guest-host interaction potential of endohedral fullerene complexes are in mutual contradiction, as far as the significance of the polarization energy contribution is concerned. This issue is elucidated through a reliable estimate of the polarization energy.

A semi-empirical analytical potential for diatomic molecules at spherical fullerenes

Abstract A simple analytical semi-empirical potential for the interaction between a diatomic molecule and a spherical fullerene is proposed. The potential assumes a spherical continuum representation for the fullerene cage. Equilibrium positions and vibrational frequencies calculated with our expression are in good agreement with ab initio computations. The model is used in a systematic and comparative analysis of the statics of the fullerene endohedral complexes of small diatomic molecules.