Unlike covalent two-dimensional (2D) materials like graphene, 2D metals have nonlayered structures due to their nondirectional, metallic bonding. While experiments on 2D metals are still scarce and challenging, densityfunctional theory (DFT) provides an ideal approach to predict their basic properties and assist in their design. However, DFT methods have rarely been benchmarked against metallic bonding at low dimensions. Therefore, to identify optimal DFT attributes for a desired accuracy, we systematically benchmark exchange-correlation functionals from LDA to hybrids and basis sets from plane waves to local basis with different pseudopotentials. With 1D chain, 2D honeycomb, 2D square, 2D hexagonal, and 3D bulk metallic systems, we compare the DFT attributes using bond lengths, cohesive energies, elastic constants, densities of states, and computational costs. Although today most DFT studies on 2D metals use plane waves, our comparisons reveal that local basis with often-used Perdew-Burke-Ernzerhof exchange correlation is quite sufficient for most purposes, while plane waves and hybrid functionals bring limited improvement compared to the greatly increased computational cost. These results ease the demands for generating DFT data for better interaction with experiments and for datadriven discoveries of 2D metals incorporating machine learning algorithms.

The discovery of two-dimensional (2D) iron monolayer in graphene pores stimulated experimental and computational material scientists to investigate low-dimensional elemental metals. There have been many advances in their synthesis, stability, and properties in the last few years. Inspired by these advancements, we investigated the electronic structure and elasticity of free-standing monolayers of group 10 elemental metals, viz. Ni, Pd, and Pt. Using density-functional theory (DFT), we explored the energetic, geometric, electronic, and elastic properties of hexagonal, honeycomb, and square lattice structures of each element, in both planar and buckled forms. Among planar configurations, the order of increasing stability is honeycomb, square, and hexagonal. In buckled form, this ordering remains the same for Pt but is reversed for Ni and Pd. Upon geometrical optimization, the extent of buckling for Pt was found to be small compared to Ni and Pd. The effect of buckling on the electronic structure was further scrutinized through the projected density of states, and it was found that highly buckled configurations derive their of states from 3D bulk, which highlights the correlation between buckled configurations and 3D bulk. For Pt in buckled square and honeycomb lattices, the density of states correlates more closely to their 2D monolayers. Regarding elasticity, the in-plane elastic constants indicate that all planar and buckled square lattices are unstable.