Cr2 in density-functional theory: approximate spin projection

The ground-state potential energy curve of Cr2 is examined using three density-functional exchange-correlation approximations: the local spin-density approximation (LSDA), the gradient-corrected BLYP approximation, and the recent B3P86 approximation incorporating a small admixture of exact exchange. An attempt is made in this work to project, at least approximately, the pure singlet energy from the usual Cr2 antiferromagnetic broken-symmetry state. The effect of this approximate spin projection is significant. Only the B3P86 functional is found to give good agreement with experiment after spin projection. Moreover, we obtain a double-well potential energy curve which has been suggested by other theoretical (non-DFT) studies and even by experiment.