COUPLING-CONSTANT DEPENDENCE OF ATOMIZATION ENERGIES

The local spin-density (LSD) functional and Perdew-Wang 91 (PW91) gene ralized gradient approximations to atomization energies of molecules a re investigated. We discuss the coupling-constant dependence of the at omization energy and why exchange errors of the functionals are greate r than exchange-correlation errors. This fact helps to justify hybrid schemes which mix some exact exchange with density functional approxim ations for exchange and correlation. It is shown that the biggest erro rs in the atomization energies occur when there is a strong interactio n between different electron pairs, which vanishes upon atomization. W e argue that the amount of exchange character of a molecular property, such as the atomization energy, depends on the property itself. We de fine an exact mixing coefficient b, which measures this exchange chara cter, and show that both LSD and PW91 typically overestimate this quan tity. Thus, nonempirical hybrid schemes which approximate this quantit y by its LSD or PW91 value typically do not improve the exchange-corre lation energy. (C) 1997 John Wiley & Sons, Inc.