Relativistic and QED corrections to the recently discovered first vibrational
2pσu​ state are presented. This state has an extremely small
nonrelativistic binding energy EB​=1.085045252(1)×10−9 a.u. Its wave
functions has a maximum at R≈100 a.u. and extends up to several
hundreds. It is shown that this state does not disappear if higher order
relativistic and QED corrections, including the Casimir--Polder effect, are
taken into account