UC San Diego

A precise and accurate description of the fundamental interactions between molecules in solution is imperative to develop models that represent chemical reactions in solutions at a predictive level. Here, we develop a set of many-body approaches which elevate the accuracy of both hybrid QM/MM and pure ab-initio methods. In order to address the boundary discontinuity problem in QM/MM simulations, where because the QM and MM regions are represented by two different potential energy surfaces, there is an energetic discontinuity at the QM/MM boundary, a generalized MB-QM model trained to the specific level of theory used in the QM region is chosen to represent the MM region. Using this scheme, the entire QM/MM system is treated under the same numerically "effective" Hamiltionian, just with different analytical representations in the QM and MM regions, in essence, describing the entire system at a full \textit{ab-initio} level of accuracy at the cost of a much cheaper QM/MM calculation. Finally, in a separate vein, after characterizing the effects of exact exchange for aqueous systems using hybrid SCAN functionals derived from the adiabatic connection formula, DC-SCAN, which evaluates the SCAN functional on the Hartree-Fock density, is developed. This density corrected functional is shown to provide gold-standard CCSD(T)/CBS level of accuracy for aqueous systems at the computational cost of a Hartree-Fock calculation, and represents the first DFT based model which is able to quantitatively represent the structural and energetic properties of water from the gas to condensed phases.