The stretching of closed-shell molecules is a qualitative problem for restricted Hartree-Fock that is usually circumvented by the use of unrestricted Hartree-Fock (UHF). UHF is well known to break the spin symmetry at the Coulson-Fischer point, leading to a discontinuous derivative in the potential energy surface and incorrect spin density. However, this is generally not considered as a major drawback. In this work, we present a set of two electron molecules which magnify the problem of symmetry breaking and lead to drastically incorrect potential energy surfaces with UHF. These molecules also fail with unrestricted density-functional calculations where a functional such as B3LYP gives both symmetry breaking and an unphysically low energy due to the delocalization error. The implications for density functional theory are also discussed.
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Research Article| October 29 2014
Qualitative breakdown of the unrestricted Hartree-Fock energy
Paula Mori-Sánchez, Aron J. Cohen; Qualitative breakdown of the unrestricted Hartree-Fock energy. J. Chem. Phys. 28 October 2014; 141 (16): 164124. https://doi.org/10.1063/1.4898860
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