J. N. Teixeira Rabelo

Calculations of the thermal expansion, cohesive energy and thermodynamic stability of a Van der Waals crystal - fullerene C60

The temperature dependence of the intermolecular distance and the cohesive energy in the high-temperature modification of C60 solid fullerene was studied on basis of the correlative method of unsymmetrized self-consistent field. The central intermolecular potential of Girifalco and its approximation by Yakub were used. We conclude about the decisive role of anharmonic effects at high temperatures. The discrepancy between the calculated and experimental values of intermolecular distance does not exceed 0.8% on the whole temperature interval. The temperature of loss of thermodynamic stability was obtained (≈ 1915 K) and a possible melting temperature was estimated (≈ 1400 K).

Anharmonic effects on thermodynamic properties of a graphene monolayer

We extend the unsymmetrized self-consistent-field method (USF) for anharmonic crystals to layered non-Bravais crystals to investigate structural, dynamical and thermodynamic properties of a free-standing graphene monolayer. In this theory, the main anharmonicity of the crystal lattice has been included and the quantum corrections are taken into account in an -expansion for the one-particle density matrix. The obtained result for the thermal expansion coefficient (TEC) of graphene shows a strong temperature dependence and agrees with experimental results by Bao et al. (Nat. Nanotechnol., 4 (2009) 562). The obtained value of TEC at room temperature (300 K) is and it becomes positive for . We find that quantum effects are significant for . The interatomic distance, effective amplitudes of the graphene lattice vibrations, adiabatic and isothermal bulk moduli, isobaric and isochoric heat capacities are also calculated and their temperature dependences are determined.

Statistical-mechanical investigation of the saturated vapor pressure, the enthalpy of sublimation and thermodynamic stability of the fcc phase of the C70 fullerite

Abstract Using the correlative method of the unsymmetrized self-consistent field for strongly anharmonic crystals and the virial expansion for a gas we have calculated the temperature dependence of the saturated vapor pressure P sat and the enthalpy of sublimation ΔH sub for the FCC modification of the fullerite C 70 . The potential of Verheijen et al. [ Science , 1992, 166 , 287] with parameters calculated on the basis of theoretical estimates by Pan et al . [ J. Phys. Chem ., 1991, 95 , 2944] and experimental data of Baker et al. [ Chem. Phys. Lett. , 1991, 184 , 182] and Vaughan et al. [ Science , 1991, 254 , 1350] have been utilized for the intermolecular forces. The temperature of loss of stability has been estimated for the two-phase crystal-vapor system. A comparison has been made between the results obtained with different sets of parameters and the values calculated earlier for the C 60 . The correspondence with available experimental data is satisfactory. Over the whole range of temperatures studied the dependence of the logarithm of the vapor pressure on the inverse of the temperature is close to linear: log P sat ≈ A − B T − CT , while ΔH sub decreases about 20%. Using the scaling law an estimate of the triple point parameters is given for the C 70 : T tr ≈ 1500 K, P tr ≈ 655 torr.

A quantum Monte Carlo study on electron correlation in all-metal aromatic clusters MAl4− (M = Li, Na, K, Rb, Cu, Ag and Au)

Using fixed-node diffusion quantum Monte Carlo (FN-DMC) simulation we investigate the electron correlation in all-metal aromatic clusters MAl4(-) (with M = Li, Na, K, Rb, Cu, Ag and Au). The electron detachment energies and electron affinities of the clusters are obtained. The vertical electron detachment energies obtained from the FN-DMC calculations are in very good agreement with the available experimental results. Calculations are also performed within the Hartree-Fock approximation, density-functional theory (DFT), and the couple-cluster (CCSD(T)) method. From the obtained results, we analyse the impact of the electron correlation effects in these bimetallic clusters and find that the correlation of the valence electrons contributes significantly to the detachment energies and electron affinities, varying between 20% and 50% of their total values. Furthermore, we discuss the electron correlation effects on the stability of the clusters as well as the accuracy of the DFT and CCSD(T) calculations in the present systems.

A quantum Monte Carlo study on electron correlation effects in small aluminum hydride clusters

Using the fixed-node diffusion quantum Monte Carlo (DMC) method, we investigate the electron correlation in several small relaxed and unrelaxed neutral, cationic, and anionic aluminum hydride clusters. We calculate the clusters total energies and use them to obtain the binding energies. Our results are in very good agreement with the available ab initio calculations and anion-photoelectron spectroscopy experiments. The calculations have also been performed in the Hartree–Fock (HF) approximation in order to analyse the impact of electron correlation. For the total atomic binding energy, i.e. the energy necessary to separate all the atoms, this impact varies from 20% up to about 50%, whereas for the electron binding energy, i.e. the energy required to detach or attach an electron to the cluster, it ranges from 1% up to 73%. The decomposition of the electron binding energies clearly shows that both charge redistribution and electron correlation are important for determining the detachment energies, whereas electrostatic and exchange interactions are responsible for the ionization potential.

Size dependences of the lattice parameter and thermal expansion coefficient of C60 fullerite nanoparticles

Abstract Using the statistical theory of structural, dynamic and thermodynamic properties of anharmonic crystals and small particles, we investigate size effects in C60 fullerite nanoparticles with fcc lattice. The intermolecular potential derived by Girifalco is utilized. We have considered three highly symmetrical forms of particles: cubical, spherical and octahedral, and calculated size dependences of their mean lattice parameters a and thermal expansion coefficients α. Influences of the surface tension and of the lattice relaxation near surfaces have been taken into account. For all three forms of nanoparticles, both a and α increase with decreasing size; that is a consequence of the anharmonicity. This effect is appreciably enhanced when the temperature increases. We also note a dominant contribution of the lattice relaxation to the size dependences.

Quantum monte carlo study of the energetics of small hydrogenated and fluoride lithium clusters.

An investigation of the energetics of small lithium clusters doped either with a hydrogen or with a fluorine atom as a function of the number of lithium atoms using fixed‐node diffusion quantum Monte Carlo (DMC) simulation is reported. It is found that the binding energy (BE) for the doped clusters increases in absolute values leading to a more stable system than for the pure ones in excellent agreement with available experimental measurements. The BE increases for pure, remains almost constant for hydrogenated, and decreases rapidly toward the bulk lithium for the fluoride as a function of the number of lithium atoms in the clusters. The BE, dissociation energy as well as the second difference in energy display a pronounced odd–even oscillation with the number of lithium atoms. The electron correlation inverts the odd–even oscillation pattern for the doped in comparison with the pure clusters and has an impact of 29%–83% to the BE being higher in the pure cluster followed by the hydrogenated and then by the fluoride. The dissociation energy and the second difference in energy indicate that the doped cluster Li3H is the most stable whereas among the pure ones the more stable are Li2, Li4, and Li6. The electron correlation energy is crucial for the stabilization of Li3H.

Electron Correlation Effects in All-Metal Aromatic Clusters: A Quantum Monte Carlo Study.

The electron correlation effects on the atomic and electronic structure of a few multiaromatic clusters XAl3– (X = Si, Ge, and Sn) are investigated using the diffusion quantum Monte Carlo method (DMC). We found that the vertical detachment energies are in very good agreement with available photoelectron spectroscopy data. The total binding energy of the clusters is dominated by the electron correlation energy contribution with about 55% of its total values. However, the binding energy gained in adding the dopant X into the Al3– unit to form the cluster XAl3– has been almost equally distributed among the contributions from Hartree–Fock (HF) and correlation energies. The resonance energy is found to be about 100 kcal/mol, which is roughly five times that of the organic aromatic compounds such as benzene. On the basis of some thermodynamical extremum principles we found that the order of decreasing stability of the clusters is SiAl3– ≈ GeAl3– > SnAl3–, the electron correlation impacting more the most stable ...

An overview of the interatomic correlation moments and the mean square relative atomic displacements in anharmonic crystals

We present an overview of a method developed for the calculation of dynamic interatomic correlations in anharmonic crystals based on the correlative method of unsymmetrized self-consistent field (CUSF). The quadratic correlation momenta and the mean square relative displacements have been calculated for one-, two- and three-dimensional classical models.

A quantum Monte Carlo study of hardwall spherical quantum dots

Using variational and diffusion quantum Monte Carlo methods, we have calculated the low- and high-spin state energies for a hardwall quantum dot model containing N ? 10 electrons. Our results are slightly lower than those obtained by Hartree?Fock and are in agreement with full configuration interaction calculations available for N = 3, 4, 5. We have observed that due to a very small difference between low- and high-spin state energies at sufficiently low electronic densities, the system might present spin-blockade effects.