Thermal smearing in DFT calculations: How small is really small? A case of La and Lu atoms adsorbed on graphene

Abstract The thermal smearing is an efficient tool to accelerate self-consistent field (SCF) convergence, and therefore widely employed in the modern computational materials science. One of the values most frequently used is 0.005 Ha. However, the fact that its equivalent temperature of 1578.9 K goes far beyond conditions of the very stability for most systems studied in the conventional materials science rises concerns on credibility of the data obtained. In the present report, we studied the effect of variable smearing values on different calculated parameters of selected lanthanide atoms (Ln = La and Lu) interacting with cluster and periodic graphene models, using Perdew-Burke-Ernzerhof (PBE) functional in conjunction with the empirical long-range dispersion correction by Grimme. In the cluster calculations for the La and Lu complexes with the supercoronene model for graphene, only the use of thermal smearing of the order of 10-4 Ha or below (down to Fermi occupancy) can yield convergent total energies, Ln… C interatomic distances, charge and spin of Ln atom, spin density and HOMO-LUMO plots. The results of periodic calculations were less sensitive to the choice of the smearing value. In this case, the smearing values at which convergent results can be obtained for total energies, Ln… C interatomic distances, charge and spin of Ln atom, as well as DOS plots are from 0.00004 to 0.001 Ha. In both cluster and periodic calculations, the value of 0.005 Ha was too high for reliable computed parameters to be obtained.