  The theory of the long-range interaction of metastable excited atomic states
with ground-state atoms is analyzed. We show that the long-range interaction is
essentially modified when quasi-degenerate states are available for virtual
transitions. A discrepancy in the literature regarding the van der Waals
coefficient C_6(2S;1S) describing the interaction of metastable atomic hydrogen
(2S state) with a ground-state hydrogen atom is resolved. In the the van der
Waals range a_0 << R << a_0/alpha, where a_0= hbar/(alpha m c) is the Bohr
radius and alpha is the fine structure constant, one finds the
symmetry-dependent result E_{2S;1S}(R) ~ (-176.75 +/- 27.98) E_h (a_0/R)^6 (E_h
denotes the Hartree energy). In the Casimir-Polder range a_0/alpha << R << hbar
c/L, where L = E(2S_{1/2})-E(2P_{1/2}) is the Lamb shift energy, one finds
E_{2S;1S}(R) ~ (-121.50 +/- 46.61) E_h (a_0/R)^6. In the the Lamb shift range R
>> hbar c/L, we find an oscillatory tail with a negligible interaction energy
below 10^(-36) Hz. Dirac-delta perturbations to the interaction are also
evaluated and results are given for all asymptotic distance ranges; these
effects describe the hyperfine modification of the interaction, or, expressed
differently, the shift of the hydrogen 2S hyperfine frequency due to
interactions with neighboring 1S atoms. The 2S hyperfine frequency has recently
been measured very accurately in atomic beam experiments.
