Abstract: Multi-reference effects can be covered by density functional theory (DFT) either implicitly via the exchange-correlation functional or explicitly via the form of the Kohn-Sham wave function. With the help of the exchange hole it is shown that the self-interaction error of the exchange functional will mimic long-range electron correlation effects if restricted Kohn-Sham theory is used. Functionals based on Slater or Becke exchange have a relatively large self-interaction error and, therefore, lead to a relatively large implicit coverage of long-range correlation, which, because of the possibility of doublecounting of electron correlation, has to be considered when using these functionals in connection with two- or multi-configurational descriptions based on ensemble DFT methods such as REKS (spin-Restricted Ensemble-referenced KS-DFT). Arguments are given that a REKS description of a multireference problem avoids a double-counting of long-range correlation effects, in particular as in this situation the self-interaction error of the exchange functional simulates more short- rather than longrange correlation effects. There is, however, no guarantee that the short-range effects are not doublecounted, namely once via the exchange and once via the correlation functional. Therefore, one should use hybrid functionals such as B3LYP in connection with multi-reference DFT methods because for hybrid functionals the self-interaction error and by this the implicit coverage of long(short)-range correlation effects is reduced due to the admixture of exact exchange. This rule applies also to broken-symmetry UDFT, which performs better with hybrid rather than GGA functionals. A way of avoiding the implicit coverage of multi-reference effects is given by the combination of wave function theory and DFT methods. The advantages and disadvantages of CAS-DFT are discussed and it is shown that an effective reduction of a double-counting of correlation effects is possible within this method.
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Cremer, D.; Filatov, M.; Polo, V.; Kraka, E.; Shaik, S. Implicit and Explicit Coverage of Multi-reference Effects by Density Functional Theory. Int. J. Mol. Sci. 2002, 3, 604-638.
Cremer D, Filatov M, Polo V, Kraka E, Shaik S. Implicit and Explicit Coverage of Multi-reference Effects by Density Functional Theory. International Journal of Molecular Sciences. 2002; 3(6):604-638.
Cremer, Dieter; Filatov, Michael; Polo, Victor; Kraka, Elfi; Shaik, Sason. 2002. "Implicit and Explicit Coverage of Multi-reference Effects by Density Functional Theory." Int. J. Mol. Sci. 3, no. 6: 604-638.