Compression of Spin-Adapted Multiconfigurational Wave Functions in Exchange-Coupled Polynuclear Spin Systems

We present a protocol based on unitary
transformations of molecular
orbitals to reduce the number of nonvanishing coefficients of spin-adapted
configuration interaction expansions. Methods that exploit the sparsity
of the Hamiltonian matrix and compactness of its eigensolutions, such
as the full configuration interaction quantum Monte Carlo (FCIQMC)
algorithm in its spin-adapted implementation, are well suited to this
protocol. The wave function compression resulting from this approach
is particularly attractive for antiferromagnetically coupled polynuclear
spin systems, such as transition-metal cubanes in biocatalysis, and
Mott and charge-transfer insulators in solid-state physics. Active
space configuration interaction calculations on N2 and
CN– at various bond lengths, the stretched square
N4 compounds, the chromium dimer, and a [Fe2S2]2– model system are presented as
a proof-of-concept. For the Cr2 case, large and intermediate
bond distances are discussed, showing that the approach is effective
in cases where static and dynamic correlations are equally important.
The [Fe2S2]2– case shows the
general applicability of the method.