## Electronic Structure Theory

The ELECTRONIC STRUCTURE THEORY department is interested in the development of ab initio methods for treating correlated electronic systems, using quantum chemistry and quantum Monte Carlo methods. These include full configuration interaction quantum Monte Carlo (FCIQMC), density matrix renormalization group methods, and many-body perturbation theories. Such methodologies are needed to accurately solve physical systems in which the ground (and excited state) electronic wavefunctions are strongly multiconfigurational (i.e. cannot be well-represented by a mean-field type wavefunction), and for which a high degree of basis-set flexibility is also necessary. Physical examples being studied in our group are superexchange antiferromagnetic coupling in cuprates, and the spin chemistry of Fe-porphyrns. In these systems, localized 3*d* orbitals on the metal centers induce strong correlation effects which must be treated on the same footing as electron hopping between the centers. A further system of general interest is the uniform electron gas, where our techniques are being deployed to compute highly accurate correlation energies. A more recent direction is in the development of time-dependent methods, i.e. time-propagation of correlated electronic systems using a stochastic propagation technique, inspired by the ground-state FCIQMC method. This allows us to compute spectral functions along the real-frequency axis, and excitation energies, at a high level of correlation.