Scattering lengths for collisions of ground state and metastable state hydrogen atoms

Scattering lengths for the collisions of ground and metastable state hydrogen atoms interacting via the B'(1) Sigma(u)(+), EF(1) Sigma(g)(+), e(3) Sigma(u)(+) and h(3) Sigma(g)(+) molecular potentials are evaluated from published ab initio Born-Oppenheimer potentials with adiabatic corrections, matched to multipole expansions at large interatomic separations. It is demonstrated that the scattering lengths are very sensitive to the connections between the ab initio potentials and their asymptotic forms and the need is shown for accurate interatomic potential calculations at greater separations than are currently available. The density-dependent shift and broadening of the 1S-2S transition frequency in atomic hydrogen caused by two-body elastic collisions are presented.