Thermodynamically stable lithium silicides and germanides from density-functional theory calculations

Density-functional-theory (DFT) calculations have been performed on the Li-Si and Li-Ge systems. Lithiated Si and Ge, including their metastable phases, play an important technological r\^ole as Li-ion battery (LIB) anodes. The calculations comprise structural optimisations on crystal structures obtained by swapping atomic species to Li-Si and Li-Ge from the X-Y structures in the International Crystal Structure Database, where X={Li,Na,K,Rb,Cs} and Y={Si,Ge,Sn,Pb}. To complement this at various Li-Si and Li-Ge stoichiometries, ab initio random structure searching (AIRSS) was also performed. Between the ground-state stoichiometries, including the recently found Li$_{17}$Si$_{4}$ phase, the average voltages were calculated, indicating that germanium may be a safer alternative to silicon anodes in LIB, due to its higher lithium insertion voltage. Calculations predict high-density Li$_1$Si$_1$ and Li$_1$Ge$_1$ $P4/mmm$ layered phases which become the ground state above 2.5 and 5 GPa respectively and reveal silicon and germanium's propensity to form dumbbells in the Li$_x$Si, $x=2.33-3.25$ stoichiometry range. DFT predicts the stability of the Li$_{11}$Ge$_6$ $Cmmm$, Li$_{12}$Ge$_7$ $Pnma$ and Li$_7$Ge$_3$ $P32_12$ phases and several new Li-Ge compounds, with stoichiometries Li$_5$Ge$_2$, Li$_{13}$Ge$_5$, Li$_8$Ge$_3$ and Li$_{13}$Ge$_4$.Comment: 10 pages, 5 figure

Ultracold atoms at unitarity within quantum Monte Carlo

Variational and diffusion quantum Monte Carlo (VMC and DMC) calculations of the properties of the zero-temperature fermionic gas at unitarity are reported. The ratio of the energy of the interacting to the non-interacting gas for a system of 128 particles is calculated to be 0.4517(3) in VMC and 0.4339(1) in the more accurate DMC method. The spherically-averaged pair-correlation functions, momentum densities, and one-body density matrices are very similar in VMC and DMC, but the two-body density matrices and condensate fractions show some differences. Our best estimate of the condensate fraction of 0.51 is a little smaller than values from other quantum Monte Carlo calculations

Hydrogen/nitrogen/oxygen defect complexes in silicon from computational searches

Point defect complexes in crystalline silicon composed of hydrogen, nitrogen, and oxygen atoms are studied within density-functional theory (DFT). Ab initio Random Structure Searching (AIRSS) is used to find low-energy defect structures. We find new lowest-energy structures for several defects: the triple-oxygen defect, {3O}, triple oxygen with a nitrogen atom, {N, 3O}, triple nitrogen with an oxygen atom, {3N,O}, double hydrogen and an oxygen atom, {2H,O}, double hydrogen and oxygen atoms, {2H,2O} and four hydrogen/nitrogen/oxygen complexes, {H,N,O}, {2H,N,O}, {H,2N,O} and {H,N,2O}. We find that some defects form analogous structures when an oxygen atom is replaced by a NH group, for example, {H,N,2O} and {3O}, and {H,N} and {O}. We compare defect formation energies obtained using different oxygen chemical potentials and investigate the relative abundances of the defects.Comment: 9 pages, 13 figure

Phospholipases D: Making Sense of Redundancy and Duplication

Why have two genes when one would suffice? Evolutionary pressure means that biology, unlike government, is generally intolerant of wasted effort. Therefore, when multiple genes exist presumably they are there to provide some benefit to the organism even if that benefit is not immediately obvious to us scientists. A recent report from Raghu and colleagues (Biosci. Rep. (2018) 38, pii: BSR20181690) [1] sheds some light on one possible reason for the existence of two Phospholipases D genes in chordates when only one is present in invertebrates