  A natural extension of the descriptors used in the Spectral Neighbor Analysis
Potential (SNAP) method is derived to treat atomic interactions in chemically
complex systems. Atomic environment descriptors within SNAP are obtained from a
basis function expansion of the weighted density of neighboring atoms. This new
formulation instead partitions the neighbor density into partial densities for
each chemical element, thus leading to explicit multi-element descriptors. For
$N_{elem}$ chemical elements, the number of descriptors increases as
$\mathcal{O}(N_{elem}^3)$, while the computational cost of the force
calculation as implemented in LAMMPS is limited to $\mathcal{O}(N_{elem}^2)$
and the favorable linear scaling in the number of atoms is retained. We
demonstrate these chemically aware descriptors by producing an interatomic
potential for indium phosphide capable of capturing high-energy defects that
result from radiation damage cascades. This new explicit multi-element SNAP
method reproduces the relaxed defect formation energies with substantially
greater accuracy than weighted-density SNAP, while retaining accurate
representation of the bulk indium phosphide properties.
