Complete basis set extrapolations for low-lying triplet electronic states of acetylene and vinylidene

Journal Article · Sat Jul 22 04:00:00 EDT 2000 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.481956· OSTI ID:20217035

Sherrill, C David ^[1]; Byrd, Edward F. C. ^[2]; Head-Gordon, Martin ^[2]

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1. School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 (United States)
2. Department of Chemistry, University of California, Berkeley, California 94720 (United States)

A recent study by Ahmed, Peterka, and Suits [J. Chem. Phys. 110, 4248 (1999)] has presented the first experimentally derived estimate of the singlet-triplet gap in the simplest alkyne, acetylene. Their value, T{sub 0}(a(tilde sign) {sup 3}B{sub 2})=28 900 cm{sup -1}, does not agree with previous theoretical predictions using the coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] method and a triple-{zeta} plus double polarization plus f-function basis set (TZ2P f ), which yields 30 500{+-}1000 cm{sup -1}. This discrepancy has prompted us to investigate possible deficiencies in this usually-accurate theoretical approach. Employing extrapolations to the complete basis set limit along with corrections for full connected triple excitations, core correlation, and even relativistic effects, we obtain a value of 30 900 cm-1 (estimated uncertainty {+-}230 cm-1), demonstrating that the experimental value is underestimated. To assist in the interpretation of anticipated future experiments, we also present highly accurate excitation energies for the other three low-lying triplet states of acetylene, a(tilde sign) {sup 3}B{sub u}(33 570{+-}230 cm{sup -1}), b(tilde sign) {sup 3}A{sub u}(36 040{+-}260 cm{sup -1}), and b(tilde sign) {sup 3}A{sub 2}(38 380{+-}260 cm{sup -1}), and the three lowest-lying states of vinylidene, X(tilde sign) {sup 1}A{sub 1}(15 150{+-}230 cm{sup -1}), a(tilde sign) {sup 3}B{sub 2}(31 870{+-}230 cm{sup -1}), and b(tilde sign) {sup 3}A{sub 2}(36 840{+-}350 cm{sup -1}). Finally, we assess the ability of density functional theory (DFT) and the Gaussian-3 method to match our benchmark results for adiabatic excitation energies of C{sub 2}H{sub 2}. (c) 2000 American Institute of Physics.

• OSTI ID: 20217035
• Journal Information: Journal of Chemical Physics, Journal Name: Journal of Chemical Physics Journal Issue: 4 Vol. 113; ISSN JCPSA6; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

74 ATOMIC AND MOLECULAR PHYSICS
ACETYLENE
ALKYNES
ELECTRONIC STRUCTURE
EXCITATION
EXPERIMENTAL DATA
THEORETICAL DATA
VINYLIDENE RADICALS