Abstract
The distinguishable cluster approximation for triple excitations has been applied to calculate thermochemical properties and excited states involving closed-shell and open-shell species, such as small molecules, 3d transition metal atoms, ozone, and an iron-porphyrin model. Excitation energies have been computed using the ΔCC approach by directly optimizing the excited states. A fixed-reference technique has been introduced to target selected spin-states for open-shell molecular systems. The distinguishable cluster approximation consistently improves coupled cluster with singles doubles and triples results for absolute and relative energies. For excited states dominated by a single configuration state function, the fixed-reference approach combined with high-level coupled-cluster methods has a comparable accuracy to the corresponding equation-of-motion coupled-cluster methods with a negligible amount of spin contamination.
Highlights
Open-shell molecular systems play an important role in numerous biological and non-biological processes
We present for the first time the DC-coupled-cluster with singles doubles and triples (CCSDT) calculations involving open-shell species and excited states
We have introduced a tailoring technique to fix the reference configuration state function (CSF) to the desired spin multiplicity, termed the fixed-reference approach
Summary
Open-shell molecular systems play an important role in numerous biological and non-biological processes. It has been shown in various studies that often one can gain accuracy without increasing the cost, which has been termed “internal correction” or “addition by subtraction.”8,12 These methods work by emulating the effects of left-out clusters through modification of the amplitude equations. The DC approximation was first applied to the doubles amplitude equations of Brueckner CC doubles (BCCD), which resulted in the Brueckner DC doubles (BDCD) method.10 Later, it was combined with single excitations and a complete orbital optimization.. Since all the quadratic doubles terms are reintroduced into the equations, this method shows poor results in the strongly correlated regime as CCSDT, but was shown to improve considerably upon CCSDT and coupled-cluster with singles and doubles (CCSD) and perturbative triples [CCSD(T)]21 in calculating the total and reaction energies of small closed-shell molecules as well as breaking single bonds.. III, various calculations using our new DC-CCSDT implementation, applicable to open-shell molecules, are presented
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