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Energy differences between frozen core and all electrons correlated.
MP2, MP3, MP4, B2PLYP, CCSD, and CCSD(T)

For KCl (Potassium Chloride)

Energies in Hartree
Methods with standard basis sets
3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p)
Moller Plesset perturbation MP2=FULL -0.005944 -0.016490 -0.002712 -0.009992 -0.009992 -0.010550 -0.155777 -0.155777 -0.020613
MP3=FULL       -0.009094   -0.015590      
Coupled Cluster CCSD(T)=FULL       -0.009230          
3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p)