BOUND-STATE SOLUTIONS OF THE 2-ELECTRON DIRAC-COULOMB EQUATION

We present a variational method for solving the two-electron Dirac-Coulomb equation. When the expectation value of the Dirac-Coulomb Hamiltonian is made stationary for all possible variations of the different components of a well-behaved trial function one obtains solutions representative of the physical bound state wave functions. The ground state wave function is derived from the application of a minimax principle. Since the trial function remains well-behaved, the method remains safe from the twin demons of variational collapse and continuum dissolution. The ground state wave function thus derived can be interpreted as a linear combination of different configurations. In particular, the admixing of intermediate states having one (two) electron(s) deexcited to a negative-energy orbital (orbitals) contributes a second-order level shift E0-(2) which can be identified with the second-order shift due to the Pauli blocking of the production of one (or two) virtual electron-positron pair(s). Thus the minimax solution corresponds to the renormalized ground state in quantum electrodynamics, with deexcitations to negative-energy orbitals taking the place of the avoidance of virtual pairs. If one extends the relativistic configuration interaction (RCI) treatment by additionally including negative-energy and mixed-energy eigenvectors of the Dirac-Hartree-Fock hamiltonian matrix in the two-electron basis, the calculated energy will be shifted from the conventional RCI value by an amount that is much smaller than E0-(2). For two-electron atoms, we have derived expressions for the all-spinor limit (delta-E) and the s-spinor limit (delta-E(s)) of this shift in leading orders. The all-spinor limit (delta-E) is of order alpha-4Z4 1/3 whereas the s-spinor limit (delta-E(s)) is of order alpha-4Z3 2/3. These leading components are related to the 1-pair component of E0-(2) in a simple way, and the relationships offer the possibility of computing energy due to virtual pairs. Numerical results are discussed