Mpller-Plesset perturbation theory second-order properties

SM calculations are broadly based on either the (i) Hartree-Fock method (ii) Post-Hartree-Fock methods like the Mpller-Plesset level of theory (MP), configuration interaction (Cl), complete active space self-consistent field (CASSCF), coupled cluster singles and doubles (CCSD) or (iii) methods based on DFT [24-27]. Since the inclusion of electron correlation is vital to obtain an accurate description of nearly all the calculated properties, it is desirable that SM calculations are carried out at either the second-order Mpller-Plesset (MP2) or the coupled cluster with single, double, and perturbative triple substitutions (CCSD(T)) levels using basis sets composed of both diffuse and polarization functions. 

Considering these two drawbacks, the use of Density Functional Theory (DFT) within its Kohn-Sham (KS) formulation is very appealing First, because the cost of a KS-DFT calculation is at most of the same order of magnitude as a Hartree-Fock one, whereas most of the description of electron correlation is taken into account, it is substantially less expensive than traditional correlated techniques. Moreover, most order n methods which are presently under development are all built on the DFT framework. Indeed, in a number of systematic validation studies , DFT has been shown successfully predicting various molecular properties, often giving results of quality comparable to or better than second order Mpller-Plesset (MP2) perturbation theory. It appears more and more evident that the use of DFT techniques could lead to reliable theoretical evaluation of ionization potentials. Moreover, the obtention of new exchange-correlation (XC) functionals is still under development, and one can expect that ionization potentials will be calculated more accurately in a close future. 

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