Leonardo Spanu

Nature and strength of interlayer binding in graphite.

We compute the interlayer bonding properties of graphite using an ab initio many-body theory. We carry out variational and diffusion quantum Monte Carlo calculations and find an equilibrium interlayer binding energy in good agreement with most recent experiments. We also analyze the behavior of the total energy as a function of interlayer separation at large distances comparing the results with the predictions of the random phase approximation.

Dielectric properties of water under extreme conditions and transport of carbonates in the deep Earth

Water is a major component of fluids in the Earth’s mantle, where its properties are substantially different from those at ambient conditions. At the pressures and temperatures of the mantle, experiments on aqueous fluids are challenging, and several fundamental properties of water are poorly known; e.g., its dielectric constant has not been measured. This lack of knowledge of water dielectric properties greatly limits our ability to model water–rock interactions and, in general, our understanding of aqueous fluids below the Earth’s crust. Using ab initio molecular dynamics, we computed the dielectric constant of water under the conditions of the Earth’s upper mantle, and we predicted the solubility products of carbonate minerals. We found that MgCO3 (magnesite)—insoluble in water under ambient conditions—becomes at least slightly soluble at the bottom of the upper mantle, suggesting that water may transport significant quantities of oxidized carbon. Our results suggest that aqueous carbonates could leave the subducting lithosphere during dehydration reactions and could be injected into the overlying lithosphere. The Earth’s deep carbon could possibly be recycled through aqueous transport on a large scale through subduction zones.

Stability of hydrocarbons at deep Earth pressures and temperatures

Determining the thermochemical properties of hydrocarbons (HCs) at high pressure and temperature is a key step toward understanding carbon reservoirs and fluxes in the deep Earth. The stability of carbon-hydrogen systems at depths greater than a few thousand meters is poorly understood and the extent of abiogenic HCs in the Earth mantle remains controversial. We report ab initio molecular dynamics simulations and free energy calculations aimed at investigating the formation of higher HCs from dissociation of pure methane, and in the presence of carbon surfaces and transition metals, for pressures of 2 to 30 GPa and temperatures of 800 to 4,000 K. We show that for T≥2,000 K and P≥4 GPa HCs higher than methane are energetically favored. Our results indicate that higher HCs become more stable between 1,000 and 2,000 K and P≥4 GPa. The interaction of methane with a transition metal facilitates the formation of these HCs in a range of temperature where otherwise pure methane would be metastable. Our results provide a unified interpretation of several recent experiments and a detailed microscopic model of methane dissociation and polymerization at high pressure and temperature.

Dissecting the Hydrogen Bond: A Quantum Monte Carlo Approach.

We present a Quantum Monte Carlo study of the dissociation energy and the dispersion curve of the water dimer, a prototype of hydrogen bonded system. Our calculations are based on a wave function which is a modern and fully correlated implementation of the Paulings valence bond idea: the Jastrow Antisymmetrised Geminal Power (JAGP) [Casula et al. J. Chem. Phys. 2003, 119, 6500-6511]. With this variational wave function we obtain a binding energy of -4.5(0.1) kcal/mol that is only slightly increased to -4.9(0.1) kcal/mol by using the Lattice Regularized Diffusion Monte Carlo (LRDMC). This projection technique allows for the substantial improvement in the correlation energy of a given variational guess and indeed, when applied to the JAGP, yields a binding energy in fair agreement with the value of -5.0 kcal/mol reported by experiments and other theoretical works. The minimum position, the curvature, and the asymptotic behavior of the dispersion curve are well reproduced both at the variational and the LRDMC level. Moreover, thanks to the simplicity and the accuracy of our variational approach, we are able to dissect the various contributions to the binding energy of the water dimer in a systematic and controlled way. This is achieved by appropriately switching off determinantal and Jastrow variational terms in the JAGP. Within this scheme, we estimate that the dispersive van der Waals contribution to the electron correlation is substantial and amounts to 1.5(0.2) kcal/mol, this value being comparable with the intermolecular covalent energy that we find to be 1.1(0.2) kcal/mol. The present Quantum Monte Carlo approach based on the JAGP wave function is revealed as a promising tool for the interpretation and the quantitative description of weakly interacting systems, where both dispersive and covalent energy contributions play an important role.

Entropy of liquid water from ab initio molecular dynamics.

We have computed the entropy of liquid water using a two-phase thermodynamic model and trajectories generated by ab initio molecular dynamics simulations. We present the results obtained with semilocal, hybrid, and van der Waals density functionals. We show that in all cases, at the experimental equilibrium density and at temperatures in the vicinity of 300 K, the computed entropies are underestimated, with respect to experiment, and the liquid exhibits a degree of tetrahedral order higher than in experiments. The underestimate is more severe for the PBE and PBE0 functionals than for several van der Waals functionals.

Solvation Properties of Microhydrated Sulfate Anion Clusters: Insights from abInitio Calculations

Sulfate-water clusters play an important role in environmental and industrial processes, yet open questions remain on their physical and chemical properties. We investigated the smallest hydrated sulfate anion clusters believed to have a full solvation shell, with 12 or 13 water molecules. We used ab initio molecular dynamics and electronic structure calculations based on density functional theory, with semilocal and hybrid functionals. At both levels of theory we found that configurations with the anion at the surface of the cluster are energetically favored compared to fully solvated ones, which are instead metastable. We show that infrared spectra of the anion with different solvation shells have similar vibrational signatures, indicating that a mixture of surface and internally solvated geometries are likely to be present in the experimental samples at low temperature. In addition, the computed electronic density of states of surface and internally solvated clusters are hardly distinguishable at finite temperature, with the highest occupied molecular orbital belonging to the anion in all cases. The equilibrium structure determined for SO(4)(2-)·(H(2)O)(13) differs from that previously reported; we find that the addition of one water molecule to a 12-water cluster modifies its hydration shell and that water-water bonds are preferred over water-anion bonds.

Ab initio investigation of the melting line of nitrogen at high pressure

Understanding the behavior of molecular systems under pressure is a fundamental problem in condensed-matter physics. In the case of nitrogen, the determination of the phase diagram and, in particular, of the melting line, are largely open problems. Two independent experiments have reported the presence of a maximum in the nitrogen melting curve, below 90 GPa, however the position and the interpretation of the origin of such maximum differ. By means of ab initio molecular-dynamics simulations based on density-functional theory and thermodynamic integration techniques, we have determined the phase diagram of nitrogen in the range between 20 and 100 GPa. We find a maximum in the melting line, related to a transformation in the liquid, from molecular

Electronic Structure of Aqueous Sulfuric Acid from First-Principles Simulations with Hybrid Functionals

{\text{N}}_{2}

Magnetism and superconductivity in the t − t ′ − J model

to polymeric nitrogen accompanied by an insulator-to-metal transition.

Theoretical investigation of methane under pressure

We carried out the first ab initio molecular dynamics simulations of aqueous sulfuric acid solutions using hybrid density functionals and a concentration (∼1 mol/L) similar to that of electrolyte solutions used in photocatalytic water splitting experiments. We found that while the semilocal functional PBE greatly overestimates the degree of dissociation of the HSO4– ion, the hybrid functional PBE0 yields results in qualitative agreement with those of recent Raman measurements. Our findings highlight the importance of using hybrid functionals in the description of anion solvation. We further analyzed the electronic structure of the solution and found that the energy of the highest occupied molecular orbital of the anion is above that of the water valence band maximum only in the case of SO42–. This indicates that SO42– may be kinetically favored, instead of HSO4–, in scavenging photoexcited holes from photoanodes in water oxidation reactions.