Higher-order Cn dispersion coefficients for hydrogen

The complete set of second-, third-, and fourth-order van der Waals coefficients Cn up to n=32 for the Hs1sd-Hs1sd 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 nø16 indicates that all the Cnø16 coefficients are accurate to 13 significant digits. The relative size of the fourth-order Cn s4d to the second-order Cn s2d coefficients is seen to increase as n increases and at n=32 the fourth-order term is actually larger.