Symposium

Mini symposium: Beyond density functional theory for predictive excitations

Watch a recording of the symposium here

A description of the electronic excitations in materials and molecules is central to understanding the key optical, transport and reactive properties. While density functional theory often describes ground state energetics to reasonable accuracy, it will often more qualitatively fail in its description of these excitations, and therefore methodological development which can go beyond DFT is of central importance for a theorists toolkit. In this mini-symposium, we will hear from three researchers at the cutting edge of this field, developing approaches for 'Beyond density functional theory for predictive excitaitons': Neepa Maitra (Rutgers University), Marc Dvorak (Aalto University), Fillipp Furche (UC Irvine).

Thursday 2 December 2021
Time: 3-5pm
Venue: Join Zoom Meeting
https://ucl.zoom.us/j/92519971654?pwd=T3M4aHM1NjJTTjA1NEczSVAza1d2UT09

Meeting ID: 925 1997 1654
Passcode: TYCSymp
Contact: George Booth

Quantum embedding for strongly-correlated systems - Marc Dvorak/Patrick Rinke - Aalto
I will introduce the concept and theory behind our new quantum embedding theory named dynamical configuration interaction (DCI) [1,2].  DCI is based on the Löwdin partitioning of the Hilbert space into an active space and a bath.  In the active space, an energy-dependent, effective Hamiltonian is solved with exact diagonalization.  The key feature of DCI to reduce the computational cost is the efficient computation of matrix elements of the effective Hamiltonian.  Matrix elements from the bath space are calculated in a quasiparticle picture based on a renormalized Hamiltonian instead of the bare many-body Hamiltonian.  We adopt a GW- and constrained RPA-based quasiparticle Hamiltonian and extend it to arbitrary excitation levels (single, double, triple, etc.) by including inter-quasiparticle interactions.  Including correlation at the quasiparticle level allows us to adopt a block-diagonal approximation in the bath space, reducing the computational cost, while maintaining good accuracy.  The final theory treats dynamic correlation at the GW level while providing a multireference treatment of the active space.  I will show ground and excited state results for dimer dissociation, small molecules, and porphyrin.  Finally, I will outline work-in-progress extensions of the theory meant to incorporate feedback from the active space to the bath via an additional level of self-consistency.  [1] Quantum embedding theory in the screened Coulomb interaction: Combining configuration interaction with GW/BSE, Phys. Rev. Materials 3, 070801(R). [2] Dynamical configuration interaction: Quantum embedding that combines wave functions and Green's functions, Phys. Rev. B 99, 115134.

Dispersion Size-Consistency - Filipp Furche - UCI
Recent benchmark calculations for supramolecular complexes with 4-206 atoms have shown that relative errors in binding energies from second-order Møller-Plesset (MP2) many-body perturbation theory (MBPT) increase systematically with with the system size at a rate of approximately 1%

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