where ∆E is the vertical energy difference between states, ϵ 0 is the vacuum permittivity, ℏ is reduced Planck constant, c is the speed of light in vacuum, η is the refractive index of the medium (1.51 for TPA) and μ is the electric transition dipole moment operator. The expectation values of this operator are computed between eigenstates of the Hamiltonian including the spin-orbit term as implemented in Molcas, allowing to estimate the transition dipole moments between the ground state and singlet and triplet excited states.