Hyperfine Pressure Shift for Interacting Atoms: Pitfalls of a Variational Procedure

Abstract

In a recent Letter on the hyperfine shifts in HeH the conclusion was reached that the hyperfine shift is never negative, even at the largest separations. This conclusion, which is in disagreement with fundamental ideas regarding the role of the van der Waals interaction in hyperfine pressure shifts, is traced to an uncritical use of Brillouin's theorem. Upon making the necessary corrections, we obtain a long-range negative shift of proper magnitude.