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25.6. MULTIPLE EXCITATIONS                                         189     190                                                   CHAPTER 25. EXOTICA
       into fragments, Eq. (25.5) includes correlated fluctuations on the two separated pieces.
       While in principle all this is included in the exact functional, in practice TDDFT provides
       a natural methodology for modelling these fluctuations. Simple ALDA approximations for
       polarizabilities of atoms and molecules allow C 6 coefficients to be accurately calculated by
       this method, and attempts exist to generate entire molecular enery curves, from the covalent
       bond distance to dissociation, including van der Waals. Such calculations are much more
       demanding than simple self-consistent ground-state calculations, but may well be worth the
       payoff.


       25.6  Multiple excitations
       25.7  Exact conditions
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