Role Of Electron Correlation Effects Research Articles

Bond length alternation in cyclic polyenes. VI. Coupled cluster approach with wannier orbital basis

AbstractThe role of electron correlation effects on the bond‐length alternation in linear metalliclike systems, as modeled by cyclic polyenes CNHN, N = 2n = 4v + 2, v = 1,2,…, is examined using the coupled cluster approach in the localized Wannier basis formalism. A recently developed approximate coupled pair approach which accounts for connected quadruply excited clusters is employed together with various truncation schemes for the localized doubly‐excited cluster components. It is found that for the physical value of the coupling constant, the electron correlation has only a very slight effect on the potential energy curves, yielding almost the same values for both the magnitude of the bond‐length alternation and for the stabilization energy relative to the symmetric equidistant structures as the restricted Hartree‐Fock theory. This is in contrast to a strongly correlated region where the correlation effects stabilize the undistorted non‐alternating structures. Different mechanisms of the bond‐length alternation or Peierl's distortion as implied by a simple Hückel Hamiltonian and by the Pariser‐Parr‐Pople Hamiltonian models are also pointed out.

Satellite structure in atomic spectra. III. The L X-ray emission spectrum of argon

For pt.II see ibid., vol.15, no.2, p.219-31 (1982). The satellite structure in the argon L X-ray emission spectrum has been calculated using correlated initial- and final-state wavefunctions based upon a frozen-core model for each state developed previously. Strong correlation effects in the final state give rise to intense satellite structure similar to the photoelectron satellites seen in valence-shell ionisation. The intensities differ from the photoelectron intensities as a result of relaxation in the transition process. Final-state correlation satellites investigated in this work contribute a total intensity equal to 84% of the diagram line intensity to the spectrum. Multiply-excited correlation satellites also exist and contribute a total intensity estimated to be 6.0% of the diagram line intensity for the initial excited-state populations chosen in this study. A few initial-state correlation satellites are also predicted with this distribution contributing about 0.3% intensity to the spectrum. The theoretical predictions are compared with current experimental work. Evidence is presented that the incident energy dependence of main-line to satellite-line intensities recently reported by Bonnet et al. (1980) is not due to variations in final-state correlation satellite intensities as originally proposed by these workers. The present paper highlights the important role of electron correlation effects in determining X-ray satellite structure and provides an insight into the complexity of the calculations required.