A Ranked-Orbital Approach to Select Active Spaces for High-Throughput Multireference Computation

The
past decade has seen a great increase in the application of
high-throughput computation to a variety of important problems in
chemistry. However, one area which has been resistant to the high-throughput
approach is multireference wave function methods, in large part due
to the technicalities of setting up these calculations and in particular
the not always intuitive challenge of active space selection. As we
look toward a future of applying high-throughput computation to all
areas of chemistry, it is important to prepare these methods for large-scale
automation. Here, we propose a ranked-orbital approach to select active
spaces with the goal of standardizing multireference methods for high-throughput
computation. This method allows for the meaningful comparison of different
active space selection schemes and orbital localizations, and we demonstrate
the utility of this approach across 1120 multireference calculations
for the excitation energies of small molecules. Our results reveal
that it is helpful to distinguish the method used to generate orbitals
from the method of ranking orbitals in terms of importance for the
active space. Additionally, we propose our own orbital ranking scheme
that estimates the importance of an orbital for the active space through
a pair-interaction framework from orbital energies and features of
the Hartree–Fock exchange matrix. We call this new scheme the
“approximate pair coefficient” (APC) method and we show
that it performs quite well for the test systems presented.