Adapting UFF4MOF for Heterometallic Rare-Earth Metal–Organic Frameworks

Heterometallic metal–organic
frameworks based on rare-earth
metals (RE-MOFs) have potential in a number of applications where
energy transfer between nearby metal atoms is required. This observation
implies that it is important to understand the level of local mixing
that is achieved between metals of different types during synthesis
of RE-MOFs. Density functional theory calculations can give quantitative
information on the relative energy of different configurations of
RE-MOFs, but these calculations cannot be applied to the full range
of medium- and long-range orderings that are possible in heterometallic
materials. This limitation can be overcome using force field (FF)-based
calculations if appropriate FFs are available. We show that an existing
generic FF for MOFs, UFF4MOF, does not accurately predict energies
of mixing in heterometallic Nd/Yb MOFs and introduce a modified FF
to address this shortcoming. The resulting FF is used to explore metal
orderings in large simulation volumes for a Nd/Yb MOF, illustrating
the complexities that can arise in the structure of heterometallic
RE-MOFs.