Most of the structs in this package are now quite efficient, but I see one more potential opportunity: Atom's coords field is a Vector{Float64}. Assuming this will always be a 3-vector, one option would be to create a Coordinates type. MIToS does this already, although their array type is immutable. Is this kind of change something worth considering? Would it need to be mutable? I'm thinking of methods like applytransform!.