The magnetic hyperfine splitting of the ${}_{}{}^2{}_{}{}^{}S$ ground state of the lithium atom is calculated. It is shown that the discrepancy between experiment and the value calculated in the traditional Hartree-Fock approximation can be accounted for quantitatively by the exchange polarization effect, which distorts one $1s$ orbital relative to the other. The present calculation obtains a value within one percent of the experimental value. A general procedure is proposed for evaluating operators that do not commute with the Hamiltonian, when approximate variational methods must be used.

• Received 30 November 1959

DOI:https://doi.org/10.1103/PhysRev.118.681