New developments and applications in quantum chemistry: Fragment-based models for large molecular systems and structure-reactivity relationship of small transition metal chalcogenide cluster

The last three decades have seen dramatic progress in the development and application of ab initio quantum chemical methods. However, as the molecule gets larger, it becomes computationally prohibitive to treat the entire molecule with accurate and reliable theoretical models. We present our research towards solving this problem by extending the power and applicability of highly accurate ab initio methods for the treatment of large molecular systems. We have made an effort to propose new techniques involving "Fragment-based Quantum Chemical Methods" that partition a large "otherwise impossible-to-tackle problem" into a collection of small "computationally tractable problems". This presentation will include a discussion of two recent models that we have proposed. The first is a simple, but efficient, method named "Dimers-Of-Dimers (DOD)" for the treatment of large water clusters leveraging the connectivity information of the network of hydrogen bonds responsible for holding water molecules together. The second is a robust and highly intuitive method named "Grid-Adapted-Manybody-Approach (GAMA)" with the goal to make fragment-based methods more black-box type, providing a reasonable balance between cost and accuracy for large calculations. Illustrative applications of these methods on complex systems such as polypeptides and water clusters will be presented with the goal of predicting their absolute and relative energies with accurate electronic structure methods. Finally, we also discuss potential applications of these models in Fragment Based Drug Design (FBDD) in the pharmaceutical industry.