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The Hubbard model, which is widely used in physics but is mostly unfamiliar to chemists, provides an attractive yet simple model for chemistry beyond the self consistent field molecular orbital approximation. The Hubbard model adds an effective electron-electron repulsion when two electrons occupy the same atomic orbital to the familiar Hckel Hamiltonian. Thus it breaks the degeneracy between excited singlet and triplet states and allows an explicit treatment of electron correlation. We show how to evaluate the parameters of the model from high-level ab initio calculations on two-atom fragments and then to transfer the parameters to large molecules and polymers where accurate ab initio calculations are difficult or impossible. The recently developed MS-RASPT2 method is used to generate accurate potential energy curves for ethene as a function of carbon-carbon bond length, which are used to parameterize the model for conjugated hydrocarbons. Test applications to several conjugatedaromatic molecules show that even though the model is very simple, it is capable of reasonably accurate predictions for bond lengths, and predicts molecular excitation energies in reasonable agreement with those from the MS-RASPT2 method. © 2011 American Institute of Physics.This work was supported by the Robert A. Welch Foundation through Grant No. BD-0046, by projects CTQ2007-61260, CTQ2010-14892, and CSD2007-0010 Consolider-Ingenio in Molecular Nanoscience of the Spanish MICINN/FEDER, by project MODELICO-CM S2009ESP-1691 of Comunidad de Madrid, and MTKD-CT-2006-042488, FP7-ICT-2009-4-248909-LIMA, FP7-ICT-2009-4-248855-N4E, and NMP4- SL-2008-213669-ENSEMBLE of the European Union.Peer Reviewe

A distance-dependent parameterization of the extended Hubbard model for conjugated and aromatic hydrocarbons derived from stretched ethene