The long-range interaction between hydrogen atoms

The second-order perturbation energy of two interacting atoms may be expressed as a power series in the reciprocal internuclear distance. For two normal hydrogen atoms the energy has been calculated as E 2 ( R ) = - (1.300 × 10) R -6 - (2 488 × 10 2 ) R -8 - (6 571 × 10 3 ) R -10 - (2 430 × 10 5 ) R -12 - (1 212 × 10 7 ) R -14 - (7.875 × 10 8 ) R -16 - (6 469 × 10 10 ) R -18 - (6.557 × 10 12 ) R -20 -...ryd where the coefficients are believed to be accurate to two units in the fourth significant place.