Accurate Thermochemistry with Multireference Methods: A Stress Test for Internally Contracted Multireference Coupled-Cluster Theory

The internally contracted multireference coupled-cluster
method
with single, double and perturbative triple excitations, icMRCCSD(T),
was tested for its performance in the context of computational high-accuracy
thermochemistry. The results were gauged against the standard single-reference
coupled-cluster hierarchy with up to 5-fold excitations. The test
set comprised of a selection of first-row dinuclear compounds and
the three 3d-transition metal compounds MnH, FeH, and CoH. The results
revealed two problems with the current formulation of icMRCCSD(T).
First, the choice of the Dyall Hamiltonian as the zeroth-order Hamiltonian,
which leads to a biased description of the different orbital subspaces
and particularly poor results for the atomic correlation energies,
and second, the tendency to overestimate the perturbative correction
for triply excited clusters, in particular in the presence of open
shells and correspondingly low orbital-energy gaps. The two problems
could be solved by resorting to the effective Fock operator as zeroth-order
Hamiltonian and by adopting a modified amplitude equation that includes
terms quadratic in the pair clusters. A similar modification was recently
proposed by Masios et al. (Phys. Rev. Lett. 2023, 131, 186401) in the context of applying
single-reference coupled-cluster theory to systems with small or vanishing
band gaps and we chose the acronym ‘(cT*) correction’
in analogy to that work. In contrast to the work of Masios et al.,
additional terms including single excitation clusters were omitted,
as these again lead to an overestimation of correlation effects in
more difficult cases. We also tested another alternative for the zeroth-order
Hamiltonian and additional higher-order corrections for the correlation
energy. These extensions did not significantly improve the results
and were also computationally more demanding. The improved icMRCCSD(cT*)F method yields very accurate results with errors, relative
to accurate benchmarks, better than 2 kJ/mol for total energies and
atomization energies for the entire set of examples considered in
this work.