A combined molecular dynamics+quantum mechanics method for investigation of dynamic effects on local surface structures

Recent advances on graphyne and its family members as membrane materials for water purification and desalination

Graphyne and its family members (GFMs) are allotropes of carbon (a class of 2D materials) having unique properties in form of structures, pores and atom hybridizations. Owing to their unique properties, GFMs have been widely utilized in various practical and theoretical applications. In the past decade, GFMs have received considerable attention in the area of water purification and desalination, especially in theoretical and computational aspects. More recently, GFMs have shown greater prospects in achieving optimal separation performance than the experimentally derived commercial polyamide membranes. In this review, recent theoretical and computational advances made in the GFMs research as it relates to water purification and desalination are summarized. Brief details on the properties of GFMs and the commonly used computational methods were described. More specifically, we systematically reviewed the various computational approaches employed with emphasis on the predicted permeability and selectivity of the GFM membranes. Finally, the current challenges limiting their large-scale practical applications coupled with the possible research directions for overcoming the challenges are proposed.

An optimized twist angle to find the twist-averaged correlation energy applied to the uniform electron gas

We explore an alternative to twist averaging in order to obtain more cost-effective and accurate extrapolations to the thermodynamic limit (TDL) for coupled cluster doubles (CCD) calculations. We seek a single twist angle to perform calculations at, instead of integrating over many random points or a grid. We introduce the concept of connectivity, a quantity derived from the nonzero four-index integrals in an MP2 calculation. This allows us to find a special twist angle that provides appropriate connectivity in the energy equation, which yields results comparable to full twist averaging. This special twist angle effectively makes the finite electron number CCD calculation represent the TDL more accurately, reducing the cost of twist-averaged CCD over N_s twist angles from N_s CCD calculations to N_s MP2 calculations plus one CCD calculation.

Exploiting the locality of periodic subsystem density-functional theory: efficient sampling of the Brillouin zone

In order to approximately satisfy the Bloch theorem, simulations of complex materials involving periodic systems are made n(k) times more complex by the need to sample the first Brillouin zone at n(k) points. By combining ideas from Kohn-Sham density-functional theory (DFT) and orbital-free DFT, for which no sampling is needed due to the absence of waves, subsystem DFT offers an interesting middle ground capable of sizable theoretical speedups against Kohn-Sham DFT. By splitting the supersystem into interacting subsystems, and mapping their quantum problem onto separate auxiliary Kohn-Sham systems, subsystem DFT allows an optimal topical sampling of the Brillouin zone. We elucidate this concept with two proof of principle simulations: a water bilayer on Pt[1 1 1]; and a complex system relevant to catalysis-a thiophene molecule physisorbed on a molybdenum sulfide monolayer deposited on top of an α-alumina support. For the latter system, a speedup of 300% is achieved against the subsystem DTF reference by using an optimized Brillouin zone sampling (600% against KS-DFT).

Oxygen vacancy formation for transient structures on the CeO2(110) surface at 300 and 750 K

Ab initio embedded-cluster calculations have been performed for the CeO2(110) surface using temperature induced structures from molecular dynamics (MD) snapshots. As a first step towards understanding how temperature induced distortions of the surface structure influence the surface oxygen reactivity, the energy cost of removing an O atom from the surface was calculated for 41 snapshots from the MD simulation at 300 K. The quantum mechanical embedded-cluster calculations show that already at 300 K the dynamics causes significant fluctuations (root mean square of 0.37 eV) in the O vacancy formation energy (Evac) while the distribution of the two excess electrons associated with the vacancy is virtually unaffected by the surface dynamics and remains localized on the two Ce ions close to the vacancy. It is also found that the quantum mechanical Evac fluctuations can be reproduced by oxygen vacancy calculations using only the relaxed shell-model force field (FF) itself and the MD geometries. Using the FF as the interaction model, the effect of raising the temperature to 750 K and the effect of doping with Ca were investigated for the oxygen vacancy formation.

Optical absorption and luminescence energies of F centers in CaO fromab initioembedded cluster calculations

We calculated the optical absorption and luminescence energies of electrons trapped at oxygen vacancies in CaO using a consistent embedded cluster method which accounts for the long-range polarization effects and partial covalence of CaO. Optical absorption and luminescence energies of neutral (F center) and positively charged (F+ center) vacancies are calculated by means of time dependent density functional theory using the B3LYP exchange-correlation density functional. Our results demonstrate that using large basis sets to describe a diffuse nature of excited states, and properly accounting for long-range polarization induced by charged and excited defect states, is crucial for accurate predictions of optical excitation and luminescence energies of these defects.