Geometrical parameters were also compared with MRCI results. The R-M-S deviations of distance and angle for heavier atoms (C, N, and O elements) are separately summarized in Table . Although relative energies were largely affected by IPEA shift, the impact on geometrical parameters was not significant. In most cases, the use of the IPEA shift changed the bond distance by just a few mÅ, but it improved some entries significantly (XMS-CASPT2 for S 1 geometry and CI12 ′ and RMS-CASPT2 for CI12 ′ ). However, for S 0 minimum, CI12, and CI12 ′ , the agreement of CASSCF was already extremely good, and the PT2 correction was significantly detrimental to the agreement. Nevertheless, the theoretical level employed for geometry optimization in Ref. 78 was not the highest, and further refinement of geometrical parameters will be desirable.