We have presented a multi-layer implementation of the equation of motion coupled-cluster method for the electron affinity, based on local and pair natural orbitals. The method gives consistent accuracy for both localized and delocalized anionic states. It results in many fold speedup in computational timing as compared to the canonical and DLPNO based implementation of the EA-EOM-CCSD method. We have also developed an explicit fragment-based approach which can lead to even higher speed-up with little loss in accuracy. The multi-layer method can be used to treat the environmental effect of both bonded and non-bonded nature on the electron attachment process in large molecules.<br>
<p>We
have re-evaluated the X40x10 benchmark for halogen bonding using conventional
and explicitly correlated coupled cluster methods. For the aromatic dimers at
small separation, improved CCSD(T)–MP2 “high-level corrections” (HLCs) cause
substantial reductions in the dissociation energy. For the bromine and iodine species,
(n-1)d subvalence correlation increases dissociation energies, and turns out to
be more important for noncovalent interactions than is generally realized. As
in previous studies, we find that the most efficient way to obtain HLCs is to
combine (T) from conventional CCSD(T) calculations with explicitly correlated
CCSD-F12–MP2-F12 differences.</p>
<div>
<div>A shortcoming of presently available fragment-based methods is that electron correlation (if included) is described at the level of individual electrons, resulting in many
redundant evaluations of the electronic relaxations associated with any given fluctuation.
A generalized variant of coupled-cluster (CC) theory is described, wherein the degrees of
freedom are fluctuations of fragments between internally correlated states. The effects of
intra-fragment correlation on the inter-fragment interaction is pre-computed and permanently folded into the effective Hamiltonian. This article provides a high-level description
of the CC variant, establishing some useful notation, and it demonstrates the advantage
of the proposed paradigm numerically on model systems. A companion article shows that
the electronic Hamiltonian of real systems may always be cast in the form demanded.
This framework opens a promising path to build finely tunable systematically improvable
methods to capture precise properties of systems interacting with a large number of other
systems.
</div>
</div>
<div>
<div>A shortcoming of presently available fragment-based methods is that electron correlation (if included) is described at the level of individual electrons, resulting in many
redundant evaluations of the electronic relaxations associated with any given fluctuation.
A generalized variant of coupled-cluster (CC) theory is described, wherein the degrees of
freedom are fluctuations of fragments between internally correlated states. The effects of
intra-fragment correlation on the inter-fragment interaction is pre-computed and permanently folded into the effective Hamiltonian. This article provides a high-level description
of the CC variant, establishing some useful notation, and it demonstrates the advantage
of the proposed paradigm numerically on model systems. A companion article shows that
the electronic Hamiltonian of real systems may always be cast in the form demanded.
This framework opens a promising path to build finely tunable systematically improvable
methods to capture precise properties of systems interacting with a large number of other
systems.
</div>
</div>
Communication: Spin densities within a unitary group based spin-adapted open-shell coupled-cluster theory: Analytic evaluation of isotropic hyperfine-coupling constants for the combinatoric open-shell coupled-cluster scheme
