The Fragment Molecular Orbital–Based Time- Dependent Density Functional Theory for Excited States in Large Systems

Time-dependent density functional theory (TDDFT)1-5 is widely used for excited state calculations, owing to its reasonable accuracy and low computational cost. TDDFT delivers excitation energies and oscillator strengths, and it requires much smaller computational resources than high-level ab initio methods such as symmetryadapted cluster configuration interaction (SAC-CI).6,7 The computational cost of TDDFT is similar to the configuration interaction with single excitations (CIS)8,9 or time-dependent Hartree-Fock (TDHF),10 but in general, TDDFT is known to give more accurate excitation energy than CIS or TDHF, because of the inclusion of the electron correlation in the density functional.3-5 Also, Furche et al.5 developed the analytic energy gradient of TDDFT and revealed that TDDFT gives better excited state geometries than CIS or TDHF.