Title: Polyvalent Machine-Learned Potential for Cobalt: from Bulk to Nanoparticles

Abstract: We present the development of a quadratic Spectral Neighbor Analysis Potential (q-SNAP) for ferromagnetic cobalt and its applications to bulk phases, surfaces, and nanoparticles. Trained on Density Functional Theory calculations using the Perdew-Burke-Ernzerhof (DFT-PBE) functional, this machine-learned potential enables simulations of large systems over extended time scales across a wide range of temperatures and pressures at near DFT accuracy. It is validated by closely reproducing the phonon dispersions of hexagonal close-packed (hcp) and face-centered cubic (fcc) Co, surface energies, and the relative stability of nanoparticles of various shapes. Thermal expansion and the melting point of Co computed with this potential are close to experimental values. Furthermore, this machine-learned potential goes beyond the capabilities of simpler N-body potentials by capturing nuanced properties such as vacancy formation energies on nanoparticle vertices. This accuracy and versatility make the potential suitable for a wide range of applications, including catalysis.