Editing SCS-MP2

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SCS-MP2 is the acronym for  Spin component scaled second-order Moller-Plesset perturbation theory. 

In recent years several investigation groups have published different variations of the MP2 method (see very interesting paper by Szabados), which according to them give results closer to the experimental values or to high level ab initio calculations. 

One of the more used of this approaches is the SCS-MP2 which uses a separate scaling for the second-order parallel spin pair correlation (alpha-alpha and beta-beta) and the anti-parallel-spin pair correlation (alpha-beta). See papers by Grimme.

Then whilst the MP2 energy is usually computed as E(MP2)= E(HF,SCF) + Ecorr,alpha-alpha + Ecorr,beta-beta +Ecorr,alpha-beta

the energy of the SCS-MP2 is expressed by

 E(SCS-MP2) =E (HF,SCF) + 1/3(Ecorr,alpha-alpha + Ecorr,beta-beta) +6/5(Ecorr,alpha-beta)

The MP2 gaussian output gives E (HF,SCF), Ecorr,alpha-alpha, Ecorr,beta-beta and Ecorr,alpha-beta and then we can obtain this energies at extra-computational cost 0 (from the MP2 of course!).

A. Szabados,  ''J. Chem. Phys.'' '''2006''', 125, 214105

S. Grimme, ''J. Chem. Phys.'' '''2003''', 118, 20, 9095 

M. Gerenkamp and S. Grimme, ''Chem. Phys. Lett.'' '''2004''', 392,229-235