B3LYP functional method

DFT methods are quite attractive as they explicitly include electronic correlation effects at much lower costs than the ab initio methods, thus offering the possibility of carrying out calculations on much larger systems. The HF method scales formally as ON where 0 is the number of occupied orbitals and N the total number of basis functions needed to describe the system. The MP2 calculations scale formally as OiV while higher orders of perturbation and other sophisticated treatments such as QCISD(T) or CCSD(T) scale even less favorably. On the other hand, hybrid functionals such as B3LYP takes into account the correlation effects at approximately the same cost as a HF calculation. The only inconvenience, as far as gas-phase acidity is concerned, resides in the fact that for some classes of compounds the accuracy of the B3LYP functional method seems to depend on the structure of the acid. 

Equilibrium structures of the 5//-bcnzotriazolo[l,2-tf]bcnzotriazol-6-ium and 6//-benzotriazolo[2,l -zdbcn/otriazol-5-ium inner salts 17 and 18, respectively, and their pyrido azaanalogs 19-22 have been studied by the density functional method B3LYP/6-31 lG(d,p) and found to have a fully conjugated planar geometry <2000CPH(254)375>. 

The keyword for a given density-functional method is formed by concatenating the symbols for its exchange and correlation functionals. For example, combining the Becke three-term (B3) exchange functional with the Lee-Yang-Parr (LYP) correlation functional leads to the popular B3LYP density functional method. 

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