## 3 Results and Discussion
### Spin Crossover
All PTMCp systems are electrically neutral. the reaction coordination (\(X\)) at the B3 LYP functional level. The reaction coordinate (\(X\)) is tuned to interpolate between one spin configuration and another spin configuration. The NEB method treats the structural change as a linear movement of all atoms and divides into six parts, and the total energies of intermediate structures are calculated. Firstly, the SCO of PMnCp from the HS to the IS is analyzed. The initial HS configuration is set at \(X=0\), while the IS structure is set at \(X=1\). The energies of the intermediate configurations in the HS and the IS are calculated. As shown in Figure 4a, when \(X\) is approximately 0.92, the two curves intersect, and the intersection point is where the SCO occurs. At this point, two spin states coexist. The energy of the LS spin state (blue dotted line) is much higher than that of the HS and the IS, and has no effect on the spin transition. The SCO behavior of HS-LS and HS-IS of PFcCp was also analyzed. The HS-IS conversion process (Figure 4b) can be successfully completed without the influence of the LS, while in the HS-LS process (Figure 4c), the energy of the IS is equal to that of the HS while lower than that of the LS when \(X\) is approximately 0.53. As the structure continues to change, the energy of IS is always smaller than that of the HS and the LS, indicating that the HS-LS transition cannot be induced only by stretching and rotation during the HS-LS process. Spin transition of IS-LS cannot be realized, because the energy of IS is much lower than that of LS. As for PCoCp, one finds LS-IS (Figure 4d) and LS-HS (Figure 4e) can both happen. Figure 3: Cp ring manipulation process. White sphere: H atom; gray sphere: C atom; green sphere: TM atom; blue sphere: N atom. Figure 2: Optimized structures of (**a**) PMnCp, (**b**) PFcCp, and (**c**) PCoCp. White sphere: H atom; gray sphere: C atom; blue sphere: N atom; purple sphere: Mn atom; red sphere: Fe atom; pink sphere: Co atom. Note that in the LS-HS process, the structure will firstly transit from the LS to the IS when \(X\) is approximately 0.46, and then from the IS to the HS when \(X\) is approximately 0.89. The SCO process of LS-IS-HS indicates that PCoCp may act as a tri-stable molecular device. We also calculated the IS-HS (Figure 4f) spin transition process of PCoCp, which realizes the transition when \(X\) is approximately 0.97. The corresponding potential barrier was 0.92 eV, which is significantly lower than that of LS-HS transition (2.70 eV). Therefore, we can also change the LS structure to the IS structure, and then to the HS configuration to achieve multistep SCO, reducing the potential barrier of the transformations. This shows that we can adjust the molecular spin states which change geometric structure by external stimulus. Our investigations provide a possibility that the SCO phenomena would occur in cyclopentadienyl metalloporphyrin complexes. In this work, we only discussed the SCO phenomena associated with the mechanical transformations (stretching and rotation). But not enough for PMnCp(LS) and PFeCp(LS), other factors such as electric field, light or temperature may help to induce the transition. The comprehensive study remains a challenge and needs methodological advancements.