Multiconfiguration Pair-Density Functional Theory for Spectroscopy and Photochemistry

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With support from the Chemical Theory, Models and Computational Methods Program in the Chemistry Division, Professors Laura Gagliardi of the University of Chicago and Donald G. Truhlar of the University of Minnesota will develop multi-configuration pair-density functional theory (MC-PDFT) and employ it to compute potential energy surfaces for ground and excited states. As collaborators, the two principal investigators are developing computational methods that will likely advance knowledge across a broad range of chemical space, from catalysis, to photochemistry, supramolecular chemistry, spectroscopy, thermochemistry, materials chemistry, biochemistry, and atmospheric chemistry. The new computational tools are expected to help elucidate phenomena identified by experimentalists and predict new chemical behavior in these areas. The work proposed here aims to demonstratively high levels of accuracy and applicability for quantum chemical methods. Moreover, the team will continue to develop codes that will be available in open-source packages to the community, including considerable effort into documentation. Additionally, the combined expertise of two senior investigators will allow students and postdocs to benefit from a wide range of ideas that will contribute to a well-rounded career preparation.Laura Gagliardi and Donald G. Truhlar are developing pair-density functional theory (MC-PDFT) with new types of wave functions, including localized active space wave functions, in order to exploit the localized character of some important kinds of strong electron correlation. This method will be advanced and employed to explore transition metal complexes involved in catalysis and excited states involved in photochemical reactions. Such systems often have a complex electronic structure that cannot be well described even to zero order by only a single electronic configuration, that is, by a single way of distributing the electrons in orbitals. MC-PDFT can be viewed as a way to combine wave function theory (WFT) and density functional theory (DFT) to take advantage of their respective strengths. In particular, the team proposes to develop MC-PDFT with the following objectives: i) Develop pair-density functional theory with new types of wave functions, including localized active space wave functions, in order to exploit the localized character of some important kinds of strong electron correlation, so as to enable the construction of accurate electronic structure calculations for extended chemical systems such as metal-organic frameworks, metal-ligand complexes, and molecules containing multiple weakly-coupled -orbital subsystems. ii) Derive and implement analytic Hessians for MC-PDFT so that one can determine whether a stationary point is a local minimum or saddle point and then calculate vibrational frequencies at this level of theory. This will allow the team and the community to use MC-PDFT as a next-generation tool to compute potential energy surfaces for ground and excited states. iii) Explore new kinds of density functionals based on the unpaired density and the first-order reduced density matrix, to improve the accuracy of the theory. iv) Apply the methods above to transition metal systems, spectroscopy, and photochemistry.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.