The CASPT2 energy was converged to 1 × 10 -8 and the forces were converged to 3 × 10 -4 . The def2-TZVPP-JKFIT basis was used in all calculations for density fitting and the so-called IPEA shift was not employed since it is not yet implemented for CASPT2 nuclear gradients. In PHVA, the Cr atoms and the first coordination sphere were displaced.