Ab Initio Quantum Theory of Molecular Electronic Structure

The key to understanding molecular electronic structure and dynamical behavior of molecules is an accurate assessment of many-electron correlation effects. Our group focuses on the development and applications of new quantum-mechanical methods that include correlation, particularly on the coupled-cluster theory and its renormalized, active-space, adaptive, externally corrected, extended, generalized, multi-reference, and response variants that allow us to study chemical reaction pathways and potential energy surfaces involving bond breaking, covalent as well as non-covalent interactions, radicals, biradicals, and other open-shell species, molecular electronic excitations, electron-transfer processes, properties in vibrationally and electronically excited states, and transition probability coefficients for various types of spectroscopy. We examine ways of achieving high-level coupled-cluster or numerically exact energetics by combining deterministic computations with stochastic wave function propagations. The analogous approaches that combine lower-level coupled-cluster computations with sequences of Hamiltonian diagonalizations and suitably designed adaptive algorithms are explored by us too. We also develop approximate coupled-pair approaches for strongly correlated systems, which can be used to model metal-insulator transitions, and local correlation coupled-cluster methods and their multi-level extensions characterized by the linear or sublinear scaling of the computational time with the system size and coarse-grain parallelism that can be applied to large molecular systems with hundreds of atoms, while preserving high accuracies canonical coupled-cluster theories offer for smaller molecules. Our goal is to design and apply robust, yet affordable, computational approaches that enable precise simulations of processes and properties relevant to molecular science, including, but not limited to, combustion, catalysis, photochemistry, and harnessing light to drive and control chemical reactivity. Our main interest is in methods that are predictive and systematically improvable, while offering high accuracy, ease of use, and relatively low computational costs compared to other quantum-chemistry approaches that aim at similar precision, so that one can use them to study chemical processes and phenomena involving complex molecular problems, in addition to smaller systems. We write our own computer codes for the standard coupled-cluster methods and their various novel variants developed in our group, which are distributed world-wide through a popular electronic structure package GΛMESS. We also maintain the "Piecuch Research Group" open-source GitHub repository at https://github.com/piecuch-group, which includes our home-grown CCT3, CCQ, CCpy, and Miniccpy software packages interfaced with GAMESS and other popular electronic structure codes, such as PSI4 and PySCF. Some of our methods are also available in NWChem and, in the original or modified form, in MRCC, Q-Chem, and ORCA.