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Keywords:

  • Rydberg orbitals;
  • semiempirical calculations;
  • ZINDO/S;
  • excitation energies;
  • hydrocarbons;
  • heterocycles

Abstract

Semiempirical methods, especially Zerner's INDO/S method, are well-established tools for theoretically investigating the low-lying valence excitations in large biologically important molecules. In recent years ab initio calculations have shown that low-lying excited states arise from both valence and Rydberg states. Using experience from ab initio calculations we explore whether Rydberg orbitals can be successfully added to semiempirical procedures. The procedure, implemented in Zerner's INDO/S method, consists of adding a shell of Rydberg orbitals at a single center on the molecule. New integral approximations are introduced to take into account that a Rydberg orbital has a much greater radial extent than the valence basis functions and allow mixing between the valence and Rydberg excited states. Parameters for the Rydberg orbitals are obtained by fitting to the experimental vertical excitation energies for ethylene and benzene while keeping the original valence orbital parameters fixed. The procedure is then evaluated by examining the vertical excitation energies computed for t-butadiene, all-trans hexatriene, cyclobutadiene, pyrrole, and furan. Overall we obtain encouraging results suggesting that Rydberg orbitals can realistically be treated by a semiempirical procedure. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003