Size Evolution of Photoabsorption Spectra of Small Clusters: A Computational Study.

Photoabsorption spectra of clusters, N=5-9, have been calculated using a diatomics-in-molecules like electronic structure model and a path-integral Monte Carlo sampling method. A qualitative change in the calculated spectra has been observed at N=9, which has been interpreted in terms of a structural transformation in the clusters consisting in a transition from trimer-like ionic cores observed for N≤7 to dimer-like ionic cores prevailing in through an intermediate state (comparable abundances of both types of ionic cores) observed in . The calculated spectra have been thoroughly compared with an earlier calculation on , , and reported from our group and data available for the same cluster sizes from an experiment.

Thermodynamics of small mercury clusters and the role of electronically excited states: a case study on Hg.

Classical Monte Carlo simulations in the isothermal-isobaric ensemble have been performed for the Hg cluster with the main emphasis paid to structural changes in this cluster induced by elevated temperature and pressure. Broad ranges of temperatures and pressures have been considered so that a comprehensive picture of the structural changes in Hg could be obtained and represented in the form of a phase diagram constructed in the temperature-pressure plane. The effect of the complex electronic structure of the cluster on its electronic ground state potential energy surface and equilibrium thermodynamics has been studied within a semi-empirical electronic structure model based on the approach.

Finite Systems under Pressure: Assessing Volume Definition Models from Parallel-Tempering Monte Carlo Simulations.

We have investigated different approaches to handling parallel-tempering Monte Carlo (PTMC) simulations in the isothermal-isobaric ensemble of molecular cluster/nanoparticle systems for predicting structural phase diagram transitions. We have implemented various methodologies that consist of treating pressure implicitly through its effect on the volume. Thus, the main problem in the simulations under nonzero pressure becomes the volume definition of the finite nonperiodic system, and we considered approaches based on the particles' coordinates.

Photoabsorption markers of pressure-induced phase changes in small mercury clusters. A case study on Hg.

Photoabsorption spectra of Hg have been calculated at various cluster temperatures and external pressures. A diatomics-in-molecules method has been used to model cluster electronic structure and classical isothermal-isobaric Monte Carlo simulations have been employed for sampling representative cluster configurations. Contributions of different structural isomers of the Hg cluster have been analyzed and related to structural transitions in the cluster, particularly those induced by an increased pressure.

Photoabsorption spectra of small mercury clusters: a computational study.

Photoabsorption spectra of small HgN clusters (N = 2-5) have been calculated using a diatomics-in-molecules interaction model and an atoms-in-molecules approach for transition probability calculations. Absorption cross-sections are provided over a broad range of photon energies, Ephot = 4.0-7.

Photodissociation of medium-sized argon cluster cations in the visible region.

Semiclassical methods for non-adiabatic dynamics simulations, based on a semiempirical diatomics-in-molecules model of intracluster interactions and the mean-field dynamical approach with the inclusion of quantum decoherence, have been used to study the photodissociation of argon cluster cations, Ar(N)(+)(N = 6-19), at E(phot) = 2.35 eV. Time periods upto t = 200 ps have been considered and abundance of ionic and neutral fragments, their time evolution and stability have been investigated and compared with available experimental data as well as earlier theoretical studies.

Phase transitions in free water nanoparticles. Theoretical modeling of [H2O]48 and [H2O]118.

Classical parallel-tempering Monte Carlo simulations of [H2O]48 and [H2O]118 have been performed in the isothermal-isobaric ensemble and a two-dimensional multiple-histogram method has been used to calculate the heat capacity of the two clusters. A semiempirical procedure is proposed for the inclusion of quantum effects and transformed heat capacity profiles are compared with state-of-the-art experimental data [C. Hock et al.

Computational investigations of the thermodynamic properties of size-selected water and Ar-water clusters: high-pressure transitions.

Classical parallel-tempering Monte Carlo simulations in the isothermal-isobaric ensemble were carried out for the (H2O)20 and Ar(H2O)20 clusters, over a wide range of temperatures (30-1000 K) and pressures (3 kPa-10 GPa) in order to study their thermodynamic properties and structural changes. The TIP4P/ice water model is employed for the water-water interactions, while both semiempirical and ab initio-based potentials are used to model the interaction between the rare-gas atoms and the water molecules. Temperature-pressure phase diagrams for these cluster systems were constructed by employing a two-dimensional multiple-histogram method.

Thermodynamics of water clusters under high pressures. A case study for (H2O)15 and (H2O)15CH4.

Thermal properties and structures of the water cluster containing fifteen molecules, either pure or doped with methane, are studied via classical parallel tempering Monte Carlo calculations in the isothermal-isobaric ensemble. The main emphasis is on structural transformations the cluster undergoes with increasing temperature and pressure. A simple TIP4P interaction model is employed for water and the unified-atom approximation with a Lennard-Jones potential is used to model the methane-water interaction.

Theoretical modeling of ionization energies of argon clusters: nuclear delocalization effects.

Temperature dependence of vertical ionization energies is modeled for small argon clusters (N ≤ 13) using classical parallel-tempering Monte Carlo methods and extended interaction models based on the diatomics-in-molecules approach. Quantum effects at the zero temperature are also discussed in terms of zero-point nuclear vibrations, either at the harmonic approximation level or at the fully anharmonic level using the diffusion Monte Carlo calculations. Both approaches lead to a considerable improvement of the theoretical predictions of argon clusters ionization energies and represent a realistic way of modeling of ionization energies for weakly bound and floppy complexes in general.

Structural changes in the water tetramer. A combined Monte Carlo and DFT study.

The heat capacity curve of the water tetramer has been calculated at various levels of theory and over a broad range of temperatures, T = 50-200 K. Parallel-tempering and multiple-histogram Monte Carlo methods have been used and combined with the Density Functional Theory calculations of intra-cluster interactions via the Boltzmann-reweighting approach. It is demonstrated that such a combination can yield well converged thermodynamic data even for a modest number of sample configurations, which makes the methodology particularly appropriate for the inclusion of quantum chemistry calculations in Monte Carlo simulations.