Density functional theory with fractional orbital occupations
Abstract
In contrast to the original KohnSham (KS) formalism, we propose a density functional theory (DFT) with fractional orbital occupations for the study of ground states of manyelectron systems, wherein strong static correlation is shown to be described. Even at the simplest level represented by the local density approximation (LDA), our resulting DFTLDA is shown to improve upon KSLDA for multireference systems, such as dissociation of H_{2} and N_{2}, and twisted ethylene, while performing similar to KSLDA for singlereference systems, such as reaction energies and equilibrium geometries. Because of its computational efficiency (similar to KSLDA), this DFTLDA is applied to the study of the singlettriplet energy gaps (ST gaps) of acenes, which are "challenging problems" for conventional electronic structure methods due to the presence of strong static correlation effects. Our calculated ST gaps are in good agreement with the existing experimental and highlevel ab initio data. The ST gaps are shown to decrease monotonically with the increase of chain length, and become vanishingly small (within 0.1 kcal/mol) in the limit of an infinitely large polyacene. In addition, based on our calculated active orbital occupation numbers, the ground states for large acenes are shown to be polyradical singlets.
 Publication:

Journal of Chemical Physics
 Pub Date:
 April 2012
 DOI:
 10.1063/1.3703894
 arXiv:
 arXiv:1201.4866
 Bibcode:
 2012JChPh.136o4104C
 Keywords:

 density functional theory;
 dissociation;
 electron correlations;
 energy gap;
 ground states;
 manybody problems;
 molecular configurations;
 polymers;
 triplet state;
 36.20.Kd;
 36.20.Hb;
 36.20.Fz;
 36.20.Ey;
 33.15.Bh;
 31.15.E;
 Electronic structure and spectra;
 Configuration;
 Constitution;
 Conformation;
 General molecular conformation and symmetry;
 stereochemistry;
 Densityfunctional theory;
 Physics  Chemical Physics;
 Condensed Matter  Materials Science;
 Physics  Computational Physics;
 Quantum Physics
 EPrint:
 accepted for publication in J. Chem. Phys., 45 pages, 21 figures, 3 tables, supplementary material not included