Density functional calculations with configuration interaction for the excited states of molecules

Configuration interaction (CI) calculations restricted to single excitations with respect to a closed-shell ground state determinant have been performed using modified CI-Hamiltonian matrix elements. Shifted molecular-orbital (MO) eigenvalues from Kohn-Sham density Functional theory (DFT) are used in the diagonal matrix elements. All Coulomb type two-electron integrals are scaled by an empirically determined factor. The approach, which is applicable to large molecules, is used with the ground state Kohn-Sham MOs expressed in extended Gaussian AO basis sets. The excited singlet and triplet states of a wide range of molecules including aromatic hydrocarbons as large as pentacene (C22H14) have been investigated. The errors of vertical excitation energies are in most cases below 0.2 eV, even for molecules for which traditional ab initio CI methods have substantial difficulties. The quality of the wavefunctions is examined by calculating the electronic circular dichroism spectra of systems with low symmetry (camphor, 4,5-dimethylphenanthrene) and good agreement with experiment is found.