Automorphic equivalence preserves the split property

We prove that the split property is a stable feature for spin chain states which are related by composition with *-automorphisms generated by power-law decaying interactions. We apply this to the theory of the ${\mathbb{Z}}_2$-index for gapped ground states of symmetry protected topological phases to show that the ${\mathbb{Z}}_2$-index is an invariant of gapped classification of phases containing fast-decaying interactions.