Optimization of Capping Potentials for Spectroscopic Parameters in Hybrid Quantum Mechanical/Mechanical Modeling Calculations

We present a capping scheme for hybrid calculations which is designed for a systematic optimization to reproduce the molecular structure, frontier bond potential, and spectroscopic properties for the quantum subsystem. Our technique is capable of reducing the perturbations of the electronic structure which are normally caused by conventional link atoms between quantum and classical regions. Specifically, we propose analytic effective core potentials with a small set of adjustable parameters, which are optimized to reproduce the full-quantum-mechanical (full-QM) properties in the direct environment of the bond cleavage. The capping potentials are conceptually simple and easy to employ in most instances without significant code modifications. They do not require any further external geometry constraints and yield also reasonable results for the potential energy surface. We benchmark these potentials for a series of chemically and biologically relevant molecules calculating NMR chemical shifts, protonation energies, and optimized geometries. Our optimized QM/mechanical modeling (MM) potentials are another step toward a realistic first-principles prediction of spectroscopic parameters in complex chemical environments using hybrid QM/MM calculations.