One-centre calculation of dispersion coefficients between ground state H atoms from pseudostate decomposition of static multipole polarizabilities

N-term pseudostate decomposition of the exact H atom static polarizabilities allows for one-centre calculation of two-body and three-body dispersion coefficients between ground state H atoms to an accuracy remarkably higher than the previously reported two-centre results. Starting from a simple power basis in the radial variable, the first-order wavefunction of the atom in the external field is expanded into a finite number N of linear pseudostates which diagonalize the matrix of the excitation energies. The expansion is rapidly convergent, and if for N = 5 the numerical results are exact to 6 significant figures, the 20-term approximation gives coefficients which are accurate to the 15th decimal place. The method can be viewed as a generalization of the Slater-Hassé-Kirkwood approximation in the context of a hypergeneralized London formula.