J. M. Gomez Llorente

The extraction of dynamics and the classical mechanical simulation of low resolution regular and chaotic spectra: HCN/HNC

The classical analog of the quantum survival probability function as well as low resolution spectra are calculated for HCN and its isomer HNC, and compared with available experimental ones and with the recent quantum mechanical SEP spectra obtained by Wyatt and co‐workers for energies up to about 30 000 cm−1. The HCN(0,v,0)A 1A‘→X 1Σ+ with the bending quantum number v=0–3, and the corresponding HNC (0,0,0) transitions were studied. It is shown that classical mechanics satisfactorily reproduces the main features of the survival probability functions and illustrates the dynamics, the results of which are the spectra. Specifically the observed spectral features are associated with the regular or chaotic motion localized in particular resonance zones. The spectroscopic agreement between classical and quantum mechanics reveals that there should be a correspondence between the regions of phase space where the classical motion is localized and the excited quantum eigenfunctions determining the SEP spectra. These...

Electronic structure and polarizabilities of icosahedral fullerenes: A Pariser–Parr–Pople approach

A Pariser–Parr–Pople approach, complemented with physical consistency criteria based on the expected molecular response to a weak electric field, has been used to predict the electronic level structure and polarizabilities of five icosahedral fullerenes in the range C60–C720. The behavior of the polarizability α as a function of the fullerene size is given by the expression α=0.75Re3, where Re is an effective molecular radius. It is argued that fullerenes would present the maximum polarizability values allowed for carbon shells, as if they were made of graphene.

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.

Saddle point resonances in a bound system with classical chaos

Abstract The saddle point region of a highly non-linear, non-separable bound Hamiltonian system, representing an isomerization process for the LiCN molecule, is found to induce a resonance-like behavior in both quantum and classical dynamics. The origin and relaxation mechanism of the resonances are given.

Semiclassical dressed states of two-level quantum systems driven by non-resonant and/or strong laser fields

Analytical expressions for the semiclassical dressed states and corresponding quasienergies are obtained for a two-level quantum system driven by a non-resonant and/or strong laser field in a coherent state. These expressions are of first order in a proper perturbative expansion, and already contain all the relevant physical information on the dynamical and spectroscopic properties displayed by these systems under such particular conditions. The influence of the laser field parameters on transition frequencies, selection rules and line intensities can be easily understood in terms of the quasienergy-level diagram and the allowed transitions between the different semiclassical dressed states.

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.