Importance of Lattice Constants in QM/MM Calculations on Metal–Organic Frameworks

Metal–organic
frameworks (MOFs) are crystalline materials
with novel physical and chemical properties. Computational simulations
have become powerful complements to experiment for understanding catalysis
in MOFs and developing new MOFs and their applications. However, due
to their relatively large and complex structures, MOFs can be burdensome
for fully quantum mechanical calculations. A combined quantum mechanical
and molecular mechanical (QM/MM) method that combines the accuracy
of fully quantum mechanical methods and the efficiency of MM methods
is therefore attractive. In this study, we employ a QM/MM method for
the study of two classes of chemical process in a MOF: the conversion
of reaction intermediates in an Ir-containing borylation catalyst
supported on MOF UiO-67 and the diffusion of a diborylated methane
molecule in the pristine UiO-67 framework. We compare the QM/MM results
with full-quantum mechanical results on large systems to validate
the accuracy of the applied QM/MM method. In the first case, we consider
a model of the entire system by partitioning it into subsystems that
interact covalently, and in the second case the subsystem interaction
is mainly steric. We observe that the QM/MM results agree with the
full-quantum mechanical results within an average of 4 kcal/mol in
the first case with strong electronic interactions and within an average
of 3 kcal/mol in the case with only noncovalent interactions. An important
lesson learned from the present study is that the quantitative results
are very sensitive to the lattice constants predicted by the MM method
used in the QM/MM calculations.