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

For HBHHBH (Diborane(4) C2V)

Energies in Hartree
Methods with standard basis sets
STO-3G 3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p) TZVP cc-pVDZ cc-pVTZ cc-pVQZ aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ aug-cc-pCVTZ
Moller Plesset perturbation MP2=FULL -0.001430 -0.002548 -0.002548 -0.002642 -0.009882 -0.010459 -0.010599 -0.034940 -0.035368 -0.013203 -0.032060 -0.004291 -0.028446 -0.051843 -0.005284 -0.034553 -0.053564 -0.089738
MP3=FULL         -0.010062   -0.014112       -0.034292 -0.004136 -0.029859         -0.094470
MP4=FULL   -0.002473     -0.010123       -0.037862     -0.004116 -0.030104   -0.005118 -0.036846   -0.095171
B2PLYP=FULL -0.000439 -0.000781 -0.000781 -0.000807 -0.002972 -0.003138 -0.003181 -0.010139 -0.010279 -0.003943 -0.009299 -0.001310 -0.008363   -0.001610 -0.010123   -0.025689
Quadratic configuration interaction QCISD(T)=FULL         -0.010110             -0.004094 -0.030070   -0.005091 -0.036810   -0.095090
QCISD(TQ)=FULL         -0.010113   -0.010826         -0.004088 -0.029828   -0.005083 -0.036661    
Coupled Cluster CCSD=FULL         -0.009986         -0.013257 -0.034341 -0.004024 -0.029784 -0.055022 -0.005007 -0.036515 -0.056929 -0.094495
CCSD(T)=FULL         -0.010112           -0.034563 -0.004094 -0.030069 -0.055555 -0.005091 -0.036807 -0.057405 -0.095070
STO-3G 3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p) TZVP cc-pVDZ cc-pVTZ cc-pVQZ aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ aug-cc-pCVTZ