Constructed to satisfy all known exact constraints and appropriate norms for a semilocal density functional, the strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation functional has shown early promise for accurately describing the electronic structure of molecules and solids. One open question is how well SCAN predicts the formation energy, a key quantity for describing the thermodynamic stability of solid-state compounds. To answer this question, we perform an extensive benchmark of SCAN by computing the formation energies for a diverse group of nearly one thousand crystalline compounds for which experimental values are known. Due to an enhanced exchange interaction in the covalent bonding regime, SCAN substantially decreases the formation energy errors for strongly-bound compounds, by approximately 50% to 110 meV/atom, as compared to the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE). However, for intermetallic compounds, SCAN performs moderately worse than PBE with an increase in formation energy error of approximately 20%, stemming from SCAN's distinct behavior in the weak bonding regime. The formation energy errors can be further reduced via elemental chemical potential fitting. We find that SCAN leads to significantly more accurate predicted crystal volumes, moderately enhanced magnetism, and mildly improved band gaps as compared to PBE. Overall, SCAN represents a significant improvement in accurately describing the thermodynamics of strongly-bound compounds.Here we consider a spin-dependent meta-GGA, just as the local spin density approximation (LSDA) is the spindependent version of LDA. Therefore, just as E xc of LSDA depends on the different spin densities (ρ ↑ and ρ ↓ ) separately, here E xc depends on the different ρ, ∇ρ, and τ for each spin channel. For brevity, we do not indicate the separate spin channels in Eq. 2.We note that, more generally, the meta-GGA rung of Jacob's ladder also includes E xc that depends on ∇ 2 ρ in addition to τ . There is evidence, however, that τ contains arXiv:1804.06914v1 [cond-mat.mtrl-sci]

The layered transition metal dichalcogenide vanadium disulfide (VS2), which nominally has one electron in the 3d shell, is potent for strong correlation physics and is possibly another realization of an effective one-band model beyond the cuprates. Here monolayer VS2 in both the trigonal prismatic and octahedral phases is investigated using density functional theory plus Hubbard U (DFT+U) calculations. Trigonal prismatic VS2 has an isolated low-energy band that emerges from a confluence of crystal field splitting and direct V-V hopping. Within spin density functional theory, ferromagnetism splits the isolated band of the trigonal prismatic structure, leading to a low-bandgap S = 1/2 ferromagnetic Stoner insulator; the octahedral phase is higher in energy. Including the on-site interaction U increases the band gap, leads to Mott insulating behavior, and for sufficiently high values stabilizes the ferromagnetic octahedral phase. The validity of DFT and DFT+U for these two-dimensional materials with potential for strong electronic correlations is discussed. A clear benchmark is given by examining the experimentally observed charge density wave (CDW) in octahedral VS2, for which DFT grossly overestimates the bond length differences compared to known experiments; the presence of CDWs is also probed for the trigonal prismatic phase. Finally, we investigate why only the octahedral phase has been observed in experiments and discuss the possibility of realizing the trigonal prismatic phase. Our work suggests trigonal prismatic VS2 is a promising candidate for strongly correlated electron physics that, if realized, could be experimentally probed in an unprecedented fashion due to its monolayer nature.

Using first-principles density functional theory, and accounting for solid-state polarization effects and electron–hole interactions, we calculate excited electronic states at interfaces between C60 and a series of functionalized boron(subphthalocyanine) molecules, a class of donor materials for organic photovoltaic (OPV) devices, and correlate energetics with their measured open-circuit voltages (V oc). For isolated donor and acceptor molecules, a staggered (type-II) interface energy alignment is predicted with an energy offset of several tenths of an electron volt, capable of promoting charge separation. The solid-state charge transfer excited state energy, E CT, obtained by including electronic polarization effects and electron–hole interactions, exhibits a near-quantitative linear relationship with V oc. E CT depends sensitively on interface morphology, resulting in a predicted 0.2–0.6 eV spread in energy for the geometries studied here. The agreement between theory and experiment provides insight into possible routes to higher V oc OPVs, and suggests that our approximate approach can enable computational design of V oc for a broad class of molecular-based OPVs.

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The ideal strength of monolayer materials possessing semimetallic, semiconducting, and insulating ground states is computed using density functional theory. Here we show that, as in graphene, a soft mode occurs at the K-point in BN, graphane, and MoS$_2$, while not in silicene. The transition is first-order in all cases except graphene. In BN and graphane the soft mode corresponds to a Kekul{\'e}-like distortion similar to that of graphene, while MoS$_2$ has a distinct distortion. The phase transitions for BN, graphane, and MoS$_2$ are not associated with the opening of a band gap, which indicates that Fermi surface nesting is not the driving force. We perform an energy decomposition that demonstrates why the soft modes at the K-point are unique and how strain drives the phonon instability