Higher-order C_n dispersion coefficients for hydrogen

The complete set of second-, third-, and fourth-order van der Waals coefficients C_n up to n=32 for the H(1s)-H(1s) dimer have been determined. They are computed by diagonalizing the nonrelativistic Hamiltonian for hydrogen to obtain a set of pseudostates that are used to evaluate the appropriate sum rules. A study of the convergence pattern for n⩽16 indicates that all the C_n⩽16 coefficients are accurate to 13 significant digits. The relative size of the fourth-order C_n⁽⁴⁾ to the second-order C_n⁽²⁾ coefficients is seen to increase as n increases and at n=32 the fourth-order term is actually larger.