Explorative computational study of the singlet fission process

Different ab initio methods, namely multi-reference and nonorthogonal configuration interaction techniques, are explored for their applicability in studying the singlet fission problem. It has been shown for 2-methyl-1,5-hexadiene that the (TT)-T-1 state can be identified using multi-reference techniques. The geometrical and vibrational properties of the (TT)-T-1 state are such that they can be approximated with those of the (TT)-T-5 state. A proof of principle is given for the calculation of the singlet fission pathway driven by nuclear motion: efficient singlet fission can take place if the (TT)-T-1 and S-1 states are close in energy with a large non-adiabatic coupling matrix element at the S-1 geometry, and the energy of the S-0 state is well below that of the (TT)-T-1 state at the (TT)-T-1 geometry. The nonorthogonal configuration interaction method was used to treat a tetracene trimer. It has been shown that the first excited states can be interpreted as delocalised states; interaction with charge-transfer base states plays an important role. The (TT)-T-1 states are localised on one pair of molecules. The electronic coupling between the diabatic S[n] and (TT)-T-1[m] states is in the meV range, confirming previous estimates. The charge-transfer base states enhance the coupling between the S[1]/S[2] and (TT)-T-1[2] excited states.