Density functional theory (DFT) methods are computationally affordable and can, in principle, be used to study the electronic energy states of the SCO complexes. Excited state DFT methods such as broken-symmetry DFT and timedependent DFT are routinely used to rationalize the energy potential and bonding effects within SCO complexes. A major complication in single-determinant methods like DFT, however, is that the Kohn-Sham determinant for many open-shell systems is not an eigenfunction of the 𝑆 ̂2 operator. Intermediate spin-state energies in such instances are not necessarily derived from the Kohn-Sham eigenfunction itself, but may instead be computed by reference to a Heisenberg-Dirac-Van Vleck Hamiltonian. Wavefunction-based methods are typically more computationally costly than DFT but may offer advantages in accuracy. For instance, domain-based pair natural orbital CCSD(T) theory has been found to be superior to DFT-based methods for determining accurate spin state energies in various systems, and spin-symmetry adapted multireference wave function theories such as CASSCF, CASPT2, and DMRG have also been demonstrated to yield accurate excitation energies. Recently, Chan and co-workers addressed the benchmark challenge of reproducing the potential energy curve of the Cr2 molecule using the DMRG-REPT methodology, which is also a multireference quantum chemical technique. While triple-decker systems similar to the above-mentioned dimer involving P5, P4, and P6 bridging ligand systems have been studied previously using DFT methods, multiconfiguration-based approaches have not yet been applied to these systems. To address the electronic structure of the dimer depicted in Figure , we here employ a combination of complete active space SCF (CASSCF) and restricted active space SCF (RASSCF) methods with multi-configuration pair-density functional theory (MC-PDFT) . This approach has been shown to compute spin state energy gaps and zerofield splitting with similar accuracies as the more computationally demanding CASPT2 method. Our multireference calculations revealed that the observed temperature-dependent magnetic susceptibility curve arises from the magnetic moment of two (or more) dimeric cation units, each with different spin states at different temperature ranges. We demonstrated that both antiferromagnetic coupling and ligand field spin transition are responsible for the spin crossover occurring over the temperature range of 150K to 12K. Our results highlight the importance of using multireference electronic structure calculations to accurately predict the correct spin states and mechanisms involved in spin crossover, particularly in cases where there is a metal dimer and single-reference methods may not be accurate enough. Further detail on the theoretical methods is provided in Section 2. Section 3 describes results from both multireference and DFT theories for single-point calculations of the dimer at the various experimental geometries obtained for different temperatures in Ref 26. Section 3.5, also quantifies the contributions of antiferromagnetic and strong ligand-field effects on the spin-crossover mechanism, and section 4 provides concluding analysis.