Improving the TDDFT calculation of low-lying excited states for polycyclic aromatic hydrocarbons using the Tamm-Dancoff approximation

Full TDDFT combined with the commonly used hybrid functional B3LYP has been known to greatly underestimate the L-1(a) excitation energies of large linear acenes. This has cast doubt on the TDDFT results for the similar L-1(a)-type state in other conjugated pi-systems. Although increasing the amount of Hartree-Fock (HT) exchange energy in the employed functional could improve the excitation energy estimation of the (1)L(a)state, it would worsen the results for another lowest-lying excited state,L-1(b). Calculations of absorption and emitting energies relative to the L-1(a) states for a series of linear acenes showed that a TDDFT scheme incorporating the Tamm-Dancoff approximation (TDDFT/TDA) could decrease the estimation errors by a factor of about 50%, but keep the levels of L-1(b) states almost unchanged. Thus, the TDDFT/TDA scheme gives an overall description for the low-lying excited states of linear acenes significantly better than the full TDDFT does. Furthermore, 16 nonlinear polycyclic aromatic hydrocarbons (PAHs) with various structures were examined to con-firm the superiority of the TDDFT/TDA over the full TDDFT in its ability to describe the L-1(a) states for conjugated pi-systems of large size. (C) 2007 Wiley Periodicals, Inc.