Number Dirac-Fock-Breit calculations

The no-pair DCB Hamiltonian (6) is used as a starting point for variational or many-body relativistic calculations [9], The procedure is similar to the nonrelativistic case, with the Hartree-Fock orbitals replaced by the four-component Dirac-Fock-Breit (DFB) functions. The spherical symmetry of atoms leads to the separation of the one-electron equation into radial and spin-angular parts [10], The radial four-spinor has the so-called large component the upper two places and the small component Q, in the lower two. The quantum number k (with k =j+ 1/2) comes from the spin-angular equation, and n is the principal quantum number, which counts the solutions of the radial equation with the same k. Defining... 

Desclaus has developed a computer code to solve the many-electron Dirac-Rock equation for atoms in a numerical self-consistent method. In this method, the relativistic Hamiltonian is approximated within the Dirac-Fock method, ignoring the two-electron Breit interaction. The Breit interaction is introduced as a first-order perturbation to energy after self-consistency is achieved. Relativistic wavefunctions and energies calculated this way are available for a number of atoms. ... 

The no-virtual-pair Dirac-Coulomb-Breit Hamiltonian, correct to second order in the fine-structure constant a, provides the framework for four-component methods, the most accurate approximations in electronic structure calculations for heavy atomie and molecular systems, ineluding aetinides. Electron correlation is taken into aeeount by the powerful coupled eluster approaeh. The density of states in actinide systems necessitates simultaneous treatment of large manifolds, best achieved by Fock-space coupled eluster to avoid intruder states, which destroy the convergence of the CC iterations, while still treating a large number of states simultaneously, intermediate Hamiltonian sehemes are employed. 

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