Ab initio calculations of molecular properties using the orbital optimized random-phase approximation method

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In this project, I will develop, implement, and use a parameter-free quantum mechanical simulation method with affordable computational cost for chemically accurate description of molecular properties. The method is based on the random phase approximation (RPA) for correlation energy. In contrast to previous RPA methods, the novel approach is independent of conventional semi-local density functional theory (DFT). The new method represents a generalization of the Kohn-Sham/Hartree-Fock scheme and is a promising tool for accurate description of weak chemical bonding, e.g., long-ranged dispersion interactions, and for correlation in isolators, semiconductors, and metal-like compounds. Using RPA, the energy contains the full Fock exchange and the RPA-based correlation energy. Like the kinetic energy in semi-local DFT, exchange and correlation are represented by orbital expressions. The orbitals construct the electron density of the molecular ground state. Within this framework the RPA correlation energy is calculated using efficient linear response techniques borrowed from time-dependent DFT. This is in contrast to single reference post-Hartree-Fock methods which make use of determinants belonging to electron excitations into virtual orbitals. Hence, RPA is a combination of exact wavefunction techniques and reasonable approximations based on DFT. So far, the orbitals are determined by semi-local DFT. These orbitals do not belong to the minimum of the RPA energy hypersurface. This makes calculations of molecular properties difficult at the RPA level. Thus, the orbitals should be derived from variation of the RPA-based total energy. For the first time, this will be done in this proposed project and will lead to a new method called orbital optimized RPA. It will be used to predict equilibrium structures and electronic excitation spectra of dispersion-bound complexes, dipole-bound anions, and metal clusters with and without adsorbed molecules.