Spurious fractional charge on dissociated atoms: Pervasive and resilient self-interaction error of common density functionals

摘要

Semilocal density functional approximations for the exchange-correlation energy can improperly dissociate a neutral molecule XY (Y not= X) to fractionally charged fragments X~(+q) ...Y~(-q) with an energy significantly lower than X~0 ...Y~0.For example,NaCl can dissociate to Na~(+0.4) ...Cl~(-0.4).Generally,q is positive when the lowest-unoccupied orbital energy of atom Y~0 lies below the highest-occupied orbital energy of atom X~0.The first 24 open sp-shell atoms of the Periodic Table can form 276 distinct unlike pairs XY,and in the local spin density approximation 174 of these display fractional-charge dissociation.Finding these lowest-energy solutions with standard quantum chemistry codes,however,requires special care.Self-interaction-corrected (SIC) semilocal approximations are exact for one-electron systems and also reduce the spurious fractional charge q.The original SIC of Perdew and Zunger typically reduces q to 0.A scaled-down SIC with better equilibrium properties sometimes fails to reduce q all the way to 0.The desideratum of "many-electron self-interaction freedom" is introduced as a generalization of the one-electron concept.