Automated Construction of Quantum–Classical Hybrid Models

We
present a protocol for the fully automated construction of quantum
mechanical (QM)–classical hybrid models by extending our previously
reported approach on self-parametrizing system-focused atomistic models
(SFAMs) [Brunken, C.; Reiher, M. J. Chem. Theory Comput. 2020, 16, 3, 1646−1665]. In this QM/SFAM
approach, the size and composition of the QM region are evaluated
in an automated manner based on first principles so that the hybrid
model describes the atomic forces in the center of the QM region accurately.
This entails the automated construction and evaluation of differently
sized QM regions with a bearable computational overhead that needs
to be paid for automated validation procedures. Applying SFAM for
the classical part of the model eliminates any dependence on pre-existing
parameters due to its system-focused quantum mechanically derived parametrization. Hence, QM/SFAM
is capable of delivering high fidelity and complete automation. Furthermore,
since SFAM parameters are generated for the whole system, our ansatz
allows for a convenient redefinition of the QM region during a molecular
exploration. For this purpose, a local reparametrization scheme is
introduced, which efficiently generates additional classical parameters
on the fly when new covalent bonds are formed (or broken) and moved
to the classical region.