Configuration Interaction with DFT orbitals: simulations in molecules

Configuration Interaction (CI) on a basis of Hartree-Fock (HF) molecular orbitals (MOs) is widely used for modelling ultrafast processes involving atoms and molecules interacting with X-rays. In many implementations for diatomic and triatomics, this method models electron correlation by coupling the molecular ground configuration with single and double excitations (CISD). For many other molecular implementations, CI simulations are expensive and often limited to single excitations (CIS), thus with no improvement to the HF ground-state wavefunction. In this oral presentation, I will discuss CIS simulations that build the molecule’s electron configurations on a basis of Kohn-Sham MOs, in order to allow the direct coupling of ground configurations with single excitations and, therefore, enable correlated ab initio computations already at the CIS level. This approach opens three prospective improvements of the quantum-chemical state-of-the-art, by i) enabling correlated ab initio calculations on large molecules, ii) flexibly modelling multi-reference wavefunctions and iii) allowing ad hoc customizations of the MO basis depending on the molecular properties and structure, due to the availability of a wide range of density functionals with different descriptions of the electron density. Subsequently, I will illustrate the performance of the approach in diverse molecules, with a particular focus on core-electron excited states that are accessible with soft X-ray sources.