Molecules can exist in different spin states (having different numbers of unpaired electrons). Predicting the lowest energy spin state for a given molecule, as well as the relative energies of the nearest spin states, is important when working with transition metals, OLEDs, and molecules with multiple unpaired electrons.
Computing spin states is straightforward. Select the possible spin states that a molecule could be in and optimize each state individually. Then compare the energies. If there are multiple low-lying energy states, it may be useful to check if the system is multireference, or if different places on the potential energy surface (PES) prefer different spins.
We have developed a simple workflow that provides a variety of established methods for computing spin-states energies that span the fast–accurate Pareto frontier. Once the results have been computed, they can be overlaid to see how the geometry changes based on the spin state.
Rowan's spin-states workflow supports a wide variety of systems. Calculations can be performed with arbitrary geometric constraints, TS optimization, solvent effects, and xTB pre-optimization to converge the results faster.
Here is the output of a spin state calculation for an octahedral Mn(Cl)6–4 complex.
The sextet state is the lowest in energy since chlorine is a weak field ligand. This contrasts with Mn(CO)6+2, where the doublet state is the lowest in energy due to CO being a strong field ligand.
