Quantum embedding methods are ideal for describing the chemistry large systems by using a higher level of theory—such as wave function (WF) theory—for the smaller active site, while still accounting for the quantum mechanical interactions with the electrons in the surrounding environment with a more computationally tractable level of theory—such as density functional theory (DFT). We have been developing a projection-based quantum embedding approach that is numerically exact for the embedding of molecular and periodic systems, that accurately accounts for the coupled time-dependent response between subsystems, and accurately and efficiently reproduce the reaction energies from canonical WF methods.<br/><br/>In projection-based embedding, the occupied orbitals of the second subsystem are projected onto the orbital space of the first using projection operators; this ensures that subsystem orbitals are mutually orthogonal and there is no need for approximate kinetic energy density functionals—which is the main source of error in traditional quantum embedding approaches.<br/><br/>We have also shown that projection-based embedding using absolutely localized subsystem orbitals and the Huzinaga projection operator, applied to WF-in-DFT embedding, reproduced canonical WF reaction energies to within 1 kcal/mol. This technique is significantly cheaper—by using a much reduced basis set for the WF region—and is more broadly applicable than previous WF-in-DFT methods. We also extended projection-based embedding to the time-dependent regime, and showed that the exact total system response—for both weakly and strongly coupled subsystems—may be obtained with a subsystem method. We are currently applying this projection-based technique to systems with periodic boundary conditions. Our results show that we can obtain the exact total system energies and densities using periodic embedding. With this method, we will be able to accurately describe chemistry at interfaces using WF methods for the active region, while still accounting for the remainder of the system using more tractable DFT methods.

Projection-based quantum embedding for molecular and periodic systems

254th National Meeting (2017)

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