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Chemical Physics Letters 393 (2004) 36–43
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A coupled cluster and full configuration interaction
study of CN and CN
Lea Thøgersen, Jeppe Olsen *
Department of Chemistry, Theoretical Chemistry, University of Aarhus, DK-8000 Aarhus, Denmark
Received 30 April 2004; in final form 27 May 2004
Abstract
Full configuration interaction (FCI) and coupled cluster (CC) calculations are carried out for the CN radical and CN using the
cc-pVDZ and an augmented cc-pVDZ basis set. In addition, CC calculations including up to quadruple excitations are carried out
using the cc-pVTZ basis. At the FCI level, the equilibrium distance is 1.1969 A, the harmonic frequency is 2020.1 cm 1 , the
electronic contribution to the atomization energy is 667 kJ/mol and the vertical electron affinity is 0.12962 E h . The contributions
from quadruple and quintuple excitations to the harmonic frequency are found to be 20 and 5 cm 1 , respectively. The quadruple
excitations give a contribution of 4 kJ/mol to the atomization energy and 0.00013 E h to the vertical electron affinity. None of the
calculations indicate that the convergence of the CC hierarchy is slower for open-shell than for closed-shell systems.
Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction
* Corresponding author. Fax: +45-861-961-99.
E-mail address: jeppe@chem.au.dk (J. Olsen).
The last decade has witnessed significant improvements
in the reliability of ab initio quantum chemical
predictions of spectroscopical and thermochemical data.
For closed shell molecules, equilibrium geometries [1],
harmonic frequencies [2] and reaction enthalpies [3,4]
may often be calculated with an accuracy that is equal to
or better than the experimental accuracy. Of central
importance for this development has been the developments
of hierarchies of basis sets [5], and CC methods
[6–8]. The coupled cluster (CC) method mostly used for
accurate calculations is the CCSD(T) method [9] which
augments the CC method including single and double
excitations (CCSD) [10] with a perturbative estimate of
triples contributions. For closed shell molecules, the
CCSD(T) method often exaggerates the contributions
from triple excitations [11]. As the signs of the triple and
quadruple corrections usually are identical, CCSD(T)
often gives results that are better than the CC method
including all single, double, and triple excitations
(CCSDT). The CCSD(T) method therefore often provides
results in surprisingly good agreement with the
much more expensive CC method including up to quadruple
excitations (CCSDTQ) [12]. Using triple-f basis
sets, the CCSD(T) method is especially accurate for
properties like internuclear distances and frequencies, as
the remaining basis-set errors and correlation errors
here usually are of opposite signs [1].
For open-shell molecules, CC methods with and
without spin-adaptation have been developed [7,13], and
the accuracy of CC calculations often matches the accuracy
obtained for closed shell molecules. In a study of
the atomization energies of 11 small molecules [2], Feller
and Sordo did not observe any systematic difference
between the accuracies obtained for closed- and openshell
molecules when the CCSDT method is used. The
performance of methods including perturbative estimates
of triple excitations as the CCSD(T) method is
less convincing for open-shell molecules. In a systematic
study of the performance of the CCSD(T) method for
the calculation of spectroscopical constants for 33 small
radicals [14], it was observed that the CCSD(T) method
did not provide constants that were significant more
accurate than those obtained with the CCSD method.
Several workers have suggested other methods combining
CCSD with the perturbative treatment of triple
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.06.001