Computational modelling also allows the differences observed experimentally between the complexes to be rationalized. Large negative values of D can be obtained due to the presence of significant SOC-induced mixing of low-lying excited states with the ground state. This is because the D tensor terms (𝐷 𝑘𝑙 ) depend on the matrix elements of the angular momentum operator, the effective SOCs (𝜉) and the energy gap between the first excited state (Q1) and the ground state (Q0) with 𝐷 𝑘𝑙 ∝ -𝜉 𝐸(𝑄 1 )-𝐸(𝑄 0 )