We first compare geometrical parameters in vacuum with experimental results in crystal. The comparison (Table ) shows that discrepancies in CASSCF geometry are mitigated when using XMS-and RMS-CASPT2. For example, the error in the N 11 -O 12 bond (N-O in the nitro group) is reduced from 0.05 Å at the CASSCF level to 0.01 Å when dynamic electron correlation is included. Similarly, an improvement is observed for the C 1 -N 14 bond (between the amino group and benzene ring). The solvent effect on geometrical parameters is minimal at the CASSCF level (Table ; additional parameters can be found in Table in the Supporting Information). The most affected bond is C 10 -N 11 (between the nitro group and benzene ring), with a difference of only 0.004 Å between vacuum and water. In contrast, at the CASPT2 level, solvent dependence becomes more pronounced: both C 10 -N 11 and C 1 -N 14 bond lengths shorten by 0.013-0.015 Å. The RMS differences between bond lengths in vacuum and water are 1.37 × 10 -3 , 7.81 × 10 -3 , and 7.07 × 10 -3 Å for CASSCF, XMS-CASPT2, and RMS-CASPT2, respectively. This indicates that CASPT2 geometry, incorporating dynamic electron correlation, is more sensitive to solvent polarity. This difference can be attributed to the larger dipole moment at the CASPT2 level. In vacuum, the dipole moments are 5.311, 6.781, and 6.378 debye at the CASSCF, XMS-CASPT2, and RMS-CASPT2 levels, respectively, indicating a more polarized electronic structure in CASPT2. In water, the dipole moments increase further, to 6.633, 9.071, and 8.460 debye. The greater dipole moments at the CASPT2 levels lead to stronger solvent effects and, consequently, larger changes in the optimized structural parameters. The difference in electronic structures between CASSCF and MS-type CASPT2 can be attributed to two main factors. The first is dynamic electron correlation, arising from electron excitations outside the active space. Although not entirely separable, the second factor involves the rotation of reference and perturbed states (diagonalization of Eqs. (34) and (37)), resulting from rotations within the internal state (CI coefficients). A comparison of the CI coefficients between CASSCF and XMS-CASPT2 for the S 0 state reveals only minor differences, underscoring the importance of dynamic electron correlation.