IPM approach to Hubbard model. Applications and limitations of restricted CI calculations to ground states of some pi networks containing fused 5-and 6-membered rings

摘要

The validity of the independent particle model (IPM) approach in treating the Hubbard model with uniform hopping parameter t and uniform on-site repulsion term LI is investigated for several U/t ratios on the ground states of some rr networks containing fused 5- and 6-membered rings with up to N = 32 pi electrons. It is shown that single and double excitation configuration interaction (SD-CI) based on a closed-shell Huckel-type reference determinant plus a correction added for the size consistency error using appropriate known correction terms leads to sensible estimates of the ground-state energy for up to U/t less than or equal to 4. The IPM approach is hence well suited for treating the chemically relevant region of U/t approximate to 1.5 - 2 for pi systems. The systems studied in this work involve several fullerene-type fragments with up to N = 16, for which a full configuration interaction (FCI) calculation is possible, and three larger systems, namely Corannulene (N = 20), the C-26 fullerene (N = 26) and the C-28(4-) fullerene anion of T-d symmetry(N = 32), for which only a limited CI involving single, double, triple, and quadruple excitations (SDTQ-CI) could be performed as a guide. SD-CI calculations on these latter systems further reproduced qualitatively correct charge densities and nearest-neighbor bond orders. Wherever possible, the obtained results were compared with those obtained by different methods present in the literature, based on Monte Carlo approaches. Further we indicated a strategy as to how to perform optimum FCI Hubbard/Pariser-Parr-Pople (PPP) calculations as far as computation time is concerned. (C) 2000 John Wiley & Sons, Inc. [References: 43]