BULK PROPERTIES FROM FINITE CLUSTER CALCULATIONS .8. BENCHMARK CALCULATIONS OF THE EFFICIENCY OF EXTRAPOLATION METHODS FOR THE HF-ENERGIES AND MP2-ENERGIES OF POLYACENES

The total energies per unit cell of both the undistorted and the Peierls-distorted polyacene polymers are computed from the respective HF/6-311G** and MP2/6-311G** finite-cluster data using 40 different extrapolation schemes. The benchmark calculations, which aim at assessing the efficiency of extrapolation methods, clearly show that the best procedure for obtaining rapidly converging bulk properties should involve computation of the energy differences, followed by rational extrapolation techniques such Wynn's rho algorithm or its iteration, both with the interpolation points x(n) = (n + 1)2, and closely related extrapolation methods, or Wynn's epsilon algorithm and its close relative, Aitken's iterated DELTA2 algorithm. (C) 1993 by John Wiley & Sons, Inc.