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 HartreeFock based extended Lagrangian and converged post-Hartree-Fock Born-Oppenheimer components, combined within the framework of 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 dynamics using post-HartreeFock electronic structure techniques, and can be very beneficial for large systems where SCF convergence may be challenging and time-consuming. Benchmarks are provided for medium-sized protonated water clusters, H9O + 4 and H13O + 6 , and poly-peptide fragments including a proline tripeptide fragment, and alanine decamer. Structural features are in excellent agreement between the hybrid approach using MP2:B3LYP fragment based electronic structure and BOMD using MP2 for the full system. Vibrational properties derived from dynamical correlation functions do show a small red-shift for the extended Lagrangian treatments, especially at higher frequencies. Strategies are discussed to improve this red-shift. The computational methodology works in parallel using both MPI and OpenMP, and shows good scaling with processor number. The timing benchmarks are provided for the alanine decamer. A powerful feature of the computational implementation is the fact that it is completely decoupled from the electronic structure package being employed and thus allows for an integrated approach that may include several different packages. These computational aspects will be further probed in future publications.