A combination of Kohn-Sham density functional theory and multi-reference configuration interaction methods

An effective Hamiltonian in a basis of spin- and space-symmetry adapted con figuration state functions (CSF), which includes information from Kohn-Sham density functional theory (DFT), is used to calculate configuration intera ction (CI) wave functions for the electronic states of molecules. The metho d emphasizes on states of multiconfigurational character which cannot be re presented by conventional DFT. The CI matrix elements are constructed empir ically by using the exact operator and corrections from DFT. Both the optim ized KS orbitals from the parent determinant and the corresponding KS poten tial from the parent state density are used. Depending on their energy gap the CI off-diagonal elements between CSF are exponentially scaled to zero t o avoid double counting of electron correlation. The selection of the most important CSF describing nondynamical correlation effects and the use of an approximate resolution of the identity (RI) for the evaluation of the two- electron integrals allows a very efficient DFT/MRCI treatment of molecules with several hundreds of electrons. As applications, the prediction of exci tation energies for singlet and triplet states of organic molecules and tra nsition metal complexes, the calculation of electronic circular dichroism s pectra and investigations of the energetics of diradicals are presented. It is found, that the new DFT/MRCI approach gives results of high accuracy (r ms errors for relative energies < 0.2 eV) comparable to those from sophisti cated ab initio treatments. (C) 1999 American Institute of Physics. [S0021- 9606(99)31837-7].