Journal
CHEMICAL PHYSICS
Volume 309, Issue 1, Pages 77-87Publisher
ELSEVIER
DOI: 10.1016/j.chemphys.2004.05.026
Keywords
polyynes; cumulenes; band gaps; excitation energies; density-functional theory; time-dependent density-functional theory; exact exchange method; localized Hartree-Fock method
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The electronic and optical properties of extended functionalized carbyne chains, polyynes and cumulenes, are investigated with the localized Hartree-Fock method, with conventional Kohn-Sham methods, and with the Hartree-Fock method. It is found that even for very long polyynes the carbon-carbon bond lengths within a polyyne alternate while for long cumulenes no carbon-carbon bond length alternation occurs. Polyynes exhibit a finite HOMO-LUMO gap even if they become very long while cumulenes are found to become metallic in the limit of long chain lengths. The geometry and the electro-optical properties of polyynes cannot be influenced significantly by simple sp-sigma-bonded end groups. The optically active (1)Sigma(u)(+) <-- X(1)Sigma(g)(+) electronic transition in polyynes is investigated by time-dependent density-functional theory (TDDFT). The known systematic underestimation of excitation energies in large chain-like systems by TDDFT methods is also found for the systems considered here. Deficiencies in the commonly used exchange-correlation kernels are identified as the main source of this shortcoming of TDDFT methods. Unphysical Coulomb self-interactions present in conventional Kohn-Sham potentials seem to not contribute significantly to the problem. (C) 2004 Elsevier B.V. All rights reserved.
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