It is well known that UHF, UMP2, UMP3 and UMP4SDTQ can properly reproduce the potential surface of H
2 dissociation (H
2®H + H) whereas RHF, RMP2, RMP3, RMP4SDTQ do not easily reproduce the proper potential surface. In this note, we demonstrate this fact because there are few textbook qualitatively representing both of them. The difference of RHF and UHF is very small in region R
H-H<1.2Å and increases along with the increasing of R
H-H>1.2Å. Corresponding to this trend, the electron correlation is large in the region R
H-H<1.2Å but rapidly decreases along with increasing of R
H-H. Finally the electron correlation is almost 0 in region R
H-H>2.0Å.
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