The combined density functional theory and multireference configuration interaction (DFT/MRCI) method was originally devised by Stefan Grimme and Mirko Waletzke. It is a well-established and efficient semi-empirical quantum chemical approach for computing molecular singlet and triplet excited-state properties. (Silva-Junior et al, DFT/MRCI review) DFT/MRCI is one of the few methods applicable to large systems that yields the correct ordering of states in extended π-systems such as polyacenes, polyenes and carotenoids where double excitations play a significant role. (Marian-2008, Lyskov-2016)
The original variant of the method encounters difficulties, however, in properly describing loosely-coupled multi-chromophore systems which play a central role in photovoltaics and in metal-free OLED emitters. For this reason, we set out redesigning and reparametrizing the Hamiltonian while maintaining the general idea of the approach. The spin-invariant redesigned DFT/MRCI Hamiltonian extends the application range of the method – in conjunction with restricted-open shell Kohn-Sham optimization of the molecular orbitals, even electronically excited doublet and quartet states can be addressed. Very recently, the redesigned Hamiltonians were combined with a novel damping function for off-diagonal matrix elements that significantly enhances the performance of the method on transition metal complexes (Heil-2018).
The DFT/MRCI program with the integrated ORCA interface, including the original (Grimme-1999) and redesigned (Lyskov-2016, Heil-2017, Heil-2018) Hamiltonians and parameterizations is available upon request here. For the academic scientific community, its use is free of charge.