To estimate the dynamic correlation effect in the employed GAS scheme in the MRCI method, the bond lengths of HgF 6 obtained by the MRCI are compared with the CCSD(T) values (cf. Table ). The symmetry of the HgF 6 molecule is fixed to O h . From the comparison between the MRCI and CCSD(T) values, the MRCI method underestimates the dynamic correlation on the bond length by 0.031-0.035 Å. From the viewpoint of dynamic correlation, the MRCI provides similar accuracy to that of the PBE0 functional of DFT . Although this deviation is similar in magnitude to the JT distortion (cf. Tables V and VII), it would not matter because the multi-reference effect would be more significant for determining the symmetry of the open-shell MF 6 systems. The MRCI and CCSD(T) methods provide the SO effects in a similar order of magnitude. This means that the coupling between the SO effect and the dynamic correlation would not affect the conclusion of our study.