Kavli Affiliate: Birgitta Whaley
| First 5 Authors: Avijit Shee, Fabian M. Faulstich, Birgitta Whaley, Lin Lin, Martin Head-Gordon
| Summary:
We develop a static quantum embedding scheme that utilizes different levels
of approximations to coupled cluster (CC) theory for an active fragment region
and its environment. To reduce the computational cost, we solve the local
fragment problem using a high-level CC method and address the environment
problem with a lower-level M{o}ller-Plesset (MP) perturbative method. This
embedding approach inherits many conceptual developments from the hybrid MP2
and CC works by Nooijen and Sherrill (J. Chem. Phys. 111, 10815 (1999), J.
Chem. Phys. 122, 234110 (2005)). We go beyond those works here by primarily
targeting a specific localized fragment of a molecule and also introducing an
alternative mechanism to relax the environment within this framework. We will
call this approach MP-CC. We demonstrate the effectiveness of MP-CC on several
potential energy curves, and a set of thermochemical reaction energies, using
CC with singles and doubles as the fragment solver, and MP2-like treatments of
the environment. The results are substantially improved by the inclusion of
orbital relaxation in the environment. Using localized bonds as the active
fragment, we also report results for ce{N=N} bond breaking in azomethane and
for the central ce{C-C} bond torsion in butadiene. We find that when the
fragment Hilbert space size remains fixed (e.g., when determined by an
intrinsic atomic orbital approach), the method achieves comparable accuracy
with both a small and a large basis set. Additionally, our results indicate
that increasing the fragment Hilbert space size systematically enhances the
accuracy of observables, approaching the precision of the full CC solver.
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