DFT-Based Multireference Diagnostics in the Solid State: Application to Metal–Organic Frameworks

When
a many-body wave function of a system cannot be captured by
a single determinant, high-level multireference (MR) methods are required
to properly explain its electronic structure. MR diagnostics to estimate
the magnitude of such static correlation have been primarily developed
for molecular systems and range from low in computational cost to
as costly as the full MR calculation itself. We report the first application
of low-cost MR diagnostics based on the fractional occupation number
calculated with finite-temperature DFT to solid-state systems. To
compare the behavior of the diagnostics on solids and molecules, we
select metal–organic frameworks (MOFs) as model materials because
their reticular nature provides an intuitive way to identify molecular
derivatives. On a series of closed-shell MOFs, we demonstrate that
the DFT-based MR diagnostics are equally applicable to solids as to
their molecular derivatives. The magnitude and spatial distribution
of the MR character of a MOF are found to have a good correlation
with those of its molecular derivatives, which can be calculated much
more affordably in comparison to those of the full MOF. The additivity
of MR character discussed here suggests the set of molecular derivatives
to be a good representation of a MOF for both MR detection and ultimately
for MR corrections, facilitating accurate and efficient high-throughput
screening of MOFs and other porous solids.