where H ( 0 ) = − ℏ 2 2 m ∇ 1 2 − ℏ 2 2 m ∇ 2 2 − e 2 r a 1 − e 2 r b 2 {\displaystyle {\mathcal {H}}^{(0)}=-{\frac {\hbar ^{2}}{2m}}\nabla _{1}^{2}-{\frac {\hbar ^{2}}{2m}}\nabla _{2}^{2}-{\frac {e^{2}}{r_{a1}}}-{\frac {e^{2}}{r_{b2}}}} and H ( 1 ) = ( e 2 R a b + e 2 r 12 − e 2 r a 2 − e 2 r b 1 ) {\displaystyle {\mathcal {H}}^{(1)}=\left({\frac {e^{2}}{R_{ab}}}+{\frac {e^{2}}{r_{12}}}-{\frac {e^{2}}{r_{a2}}}-{\frac {e^{2}}{r_{b1}}}\right)}
