A quantum Monte Carlo study of the structural and electronic properties of small cationic and neutral lithium clusters

Investigating the role of carbon doping on the structural and energetic properties of small aluminum clusters using quantum Monte Carlo

In this study, we investigate the energetics of small aluminum clusters doped with a carbon atom using several computational methods, including diffusion quantum Monte Carlo, Hartree-Fock (HF), and density functional theory. We calculate the lowest energy structure, total ground-state energy, electron population distribution, binding energy, and dissociation energy as a function of the cluster size of the carbon-doped aluminum clusters compared with the undoped ones. The obtained results show that carbon doping enhances the stability of the clusters mainly due to the electrostatic and exchange interactions from the HF contribution gain. The calculations also indicate that the dissociation energy required to remove the doped carbon atom is much larger than that required to remove an aluminum atom from the doped clusters. In general, our results are consistent with available theoretical and experimental data.

Hydrogen adsorption on lithium clusters coordinated to a gC3N4 cavity

The search of new materials having suitable characteristics to trap hydrogen for fuel applications is greatly challenging due to the stringent requirements that such materials must meet. In this sense, with the aid of computational chemistry, significant advances can be achieved. The present work explores the adsorption of hydrogen molecules by lithium clusters (Li_n, where n = 1-6) coordinated to a graphitic carbon nitride (heptazine, gC₃N₄) cavity. The study was conducted using the density functional theory (M06-2X-D3) in combination with the def2-TZVP basis set. Our results suggest that lithium atoms in the gC₃N₄-cavity can coordinate up to 10 hydrogen molecules with bond energies in the range -0.10 to -0.19 eV. The [gC₃N₄Li₅]⁺ and [gC₃N₄Li₆] systems resulted to be the most promising in terms of lithium coordination. They feature the highest stabilization energies for hydrogen adsorption. According to the calculated Gibbs free energies for these systems, H₂ adsorption remains a spontaneous process even at 400 K.

Electron correlation effects in boron clusters BQn (for Q = -1, 0, 1 and n ≤ 13) based on quantum Monte Carlo simulations

We present all-electron quantum Monte Carlo simulations on the anionic, neutral, and cationic boron clusters BQn with up to 13 atoms (Q = -1, 0, +1 and n ≤ 13). Accurate total energies of these clusters are obtained and an excellent agreement is reached with available experimental results for adiabatic and vertical detachment energies. We also perform very accurate Hartree-Fock calculations in the complete-basis-set limit where electron correlation is absent. In combination with the FN-DMC and HF-CBS results, we quantify the correlation effects and present the first attempt for a systematic investigation on the electron correlation effects in boron clusters. The obtained results show that, in general, electron correlation may contribute significantly to both the atomic and electronic structures of the boron clusters, manifested in the quantities such as the average binding energies of the clusters, atomic dissociation energies, detachment energies, and ionization potentials. For instance, the calculations indicate that the electron correlation maintains the bound state of cationic cluster B₂⁺ and it also contributes 99% of the detachment energy of the anionic cluster B₅^-.

Quantifying electron-correlation effects in small coinage-metal clusters via ab initio calculations

We investigate many-electron correlation effects in neutral and charged coinage-metal clusters Cun, Agn, and Aun (n = 1-4) via ab initio calculations using fixed-node diffusion Monte Carlo (FN-DMC) simulations, density functional theory (DFT), and the Hartree-Fock (HF) method. From very accurate FN-DMC total energies of the clusters and the HF results in the infinity large complete-basis-set limit, we obtain correlation energies in these strongly correlated many-electron clusters involving d orbitals. The obtained bond lengths of the clusters, atomic binding and dissociation energies, ionization potentials, and electron affinities are in satisfactory agreement with the available experiments. In the analysis, the electron correlation effects on these observable physical quantities are quantified by relative correlation contributions determined by the difference between the calculated FN-DMC and HF results. We show that the correlation contribution is not only significant for the quantities related to electronic structures of the coinage-metal clusters, such as electron affinity, but it is also essential for the stability of the atomic structures of these clusters. For example, the electron correlation contribution is responsible for more than 90% of the atomic binding energies of the small neutral copper clusters. We also demonstrate the orbital-occupation dependence of the correlation energy and electron pairing of the valence electrons in these coinage-metal clusters from the electron correlation-energy gain and spin-multiplicity change in the electron addition processes, which are reflected in their ionization potentials and electron affinities.