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Nonorthogonal configuration interaction is known to be a computationally inefficient approach. Recently, there has been a number of developments that have been done to improve the efficiency of calculations using nonorthogonal orbitals, as mentioned in this question.
My question is: In which situations or problems does this method outperform the 'standard' method which uses orthogonal orbitals?
Non-orthogonal configuration interaction allows for a more intuitive interpretation in terms of molecular states with strict adiabatic character. While the orbital relaxation of individual states that leads to non-orthogonality can reduce the wavefunction expansion compared to orthogonal approaches, NOCI remains a computationally expensive method. But the method parallelizes very well, and the availability of large scale HPC systems allows application to larger molecular systems of interest. The method will not 'outperform' 'standard' orthogonal approaches computationally, but its strength is in the interpretation of the components in the wavefunction expansion. NOCI is useful in studies of materials and processes in which an accurate description of individual ground and excited states and their electronic coupling is important, e.g. in singlet fission materials.
