As explained above, the wavefunction |Ψ(r, 0) at the initial time-step is represented as one of the MOs of the entire system. This choice provides a reasonable single-electron picture of the localized photo-excitation in the system. At all other time-steps, to reduce computational cost, the size of the MO basis is truncated to a smaller set containing the LUMO to LUMO+9 orbitals, which are low-lying orbitals that participate directly in the photoinduced charge transfer process.