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Hybrid Extended Lagrangian, Post-Hartree-Fock Born-Oppenheimer ab Initio Molecular Dynamics Using Fragment-Based Electronic Structure.

We present a hybrid ab initio molecular dynamics scheme that includes both DFT and Hartree–Fock-based extended Lagrangian and converged post-Hartree–Fock Born–Oppenheimer components, combined within the framework of a molecular fragmentation-based electronic structure. The specific fragmentation algorithm used here is derived from ONIOM but includes multiple, overlapping “model” systems. The interaction between the various overlapping model systems is approximated by invoking the principle of inclusion–exclusion at the chosen higher level of theory and within a “real” calculation performed at the chosen lower level of theory. Furthermore, here, the lower level electronic structure of the full system is propagated through an extended Lagrangian formalism, whereas the fragments, treated using post-Hartree–Fock electronic structure theories, are computed using the normal converged Born–Oppenheimer treatment. This conservative dynamical approach largely reduces the computational cost to approximate on-the-fly...