A simple effective ∆SCF method for computing optical gap in organic chromophores.

Photoluminescence effects in organic chromophores are of significant importance and requires precise description of low lying excited states. In this communication, we put forward an alternative time-independent DFT scheme for computing lowest single-particle excitation energy, especially for singlet excited state. This adopts a recently developed "virial"-theorem based model of singlettriplet splitting which requires a DFT calculation on closed shell ground state and a restricted open-shell triplet excited state, followed by a simple 2 e - integral evaluation. This produces vertical excitation energies in small molecules, linear and non-linear polycyclic aromatic hydrocarbon and organic dyes in comparable accuracy to the TDDFT. We also explore the functional dependency of present method with three different functionals (B3LYP, wB97X and CAM-B3LYP) for polyenes and linear acenes. A systematic comparison with literature value illustrates the validity and usefulness of the present scheme in determining optical gap with fair computational cost.