Pair structure, diffusion and pressure in liquid CuZr alloys from ab initio simulations: assessing the sensitivity to the energy cutoff

Abstract

An accurate determination of the liquid structure, dynamics and thermodynamics on the same footing from ab initio molecular dynamics requires particular attention in setting up the electronic structure calculation. Using two well established simulation codes implementing the density functional theory with modern projector augmented wave sets, extensive ab initio molecular dynamics runs have been performed on the CuZr liquid alloy, a prototype of glass-forming alloys. Simulation cells involving about 3000 electrons and energy cutoffs from 285 eV up to 640 eV, well above the values usually taken in simulations of liquid alloys have been used. The influence of the energy cutoff on structural and thermodynamic properties is examined, with a view to estimating the limiting values required for providing reliable reference data for the classical simulations with parametrized force fields. The impact on classi- cal simulations is illustrated with optimized embedded atom model potentials using the force matching method. The various data collected confirm that the ionic pair structure and the diffusion are indeed well converged for energy cutoffs larger than about 285 eV. The greater sensitivity observed for the pressure underlines the neces- sity to pay more attention to the question of incomplete basis set and the structure independent electronic contributions.

Structure and thermodynamics of liquid ruthenium and ruthenium-based alloys from ab initio and classical molecular dynamics with embedded atom model potentials

“…They are not shown in figure 2 as they are indistinguishable at the scale of the figure from the ones computed with the specialized potential. This confirms the general observation that the pair structure in liquid metals is not very sensitive to details of the EAM potential energy, in contrast with the pressure [26].…”

“…The EAM has been largely used to investigate copper based alloys-see for example [24,25] and references therein. In our recent study of the effect of the energy cutoff in CuZr binaries [26], we observed that EAM force fields with various forms of the pair terms may lead to a pair structure that agrees well with aiMD simulations. A good agreement with experiment was also found [27] when the FF was optimized using the Stillinger-Weber potential [28].…”

“…At lower density, we took ecut wfc = 1000 eV. This is much larger than the values between 100 eV and 300 eV usually considered to be sufficient for average structural quantities (see for example [44,45] for liquid metals and [18] for metallic melts), since, as shown in our recent work [26], an accurate description of the pressure requires larger cutoffs. N = 256 atoms (pure Ru) and N = 300 atoms (RuRhPd alloys) were placed in a cubic box.…”

“…ML studies without force matching exist for these systems-for thermodynamic properties of transition metal melts see for example [30,31]-but to the best of our knowledge, MLFF is virtually non-existent for multicomponent alloys of transition metals in the liquid phase. As a large number of ionic configurations, each having a large number of valence electrons must be sampled [26], the complexity is often reduced by e.g. using lower cutoffs, compositions with less electrons [18] or system sizes [23] of at most 32 atoms, as in ordered structures [32,33].…”

“…The accuracy of the clMD quantities is finally assessed by comparison with aiMD ones obtained with long enough runs. Understandably, this was limited to a few systems, due to the large energy cutoffs used [26], and the necessity to sample several thousands of ionic configurations.…”