An exact separation of the spin‐free and spin‐dependent terms of the Dirac–Coulomb–Breit Hamiltonian

The Dirac Hamiltonian is transformed by extracting the operator (σ⋅p)/2mc from the small component of the wave function and applying it to the operators of the original Hamiltonian. The resultant operators contain products of Pauli matrices that can be rearranged to give spin‐free and spin‐dependent operators. These operators are the ones encountered in the Breit–Pauli Hamiltonian, as well as some of higher order in α². However, since the transformation of the original Dirac Hamiltonian is exact, the new Hamiltonian can be used in variational calculations, with or without the spin‐dependent terms. The new small component functions have the same symmetry properties as the large component. Use of only the spin‐free terms of the new Hamiltonian permits the same factorization over spin variables as in nonrelativistic theory, and therefore all the post‐self‐consistent field (SCF) machinery of nonrelativistic calculations can be applied. However, the single‐particle functions are two‐component orbitals having a large and small component, and the SCF methods must be modified accordingly. Numerical examples are presented, and comparisons are made with the spin‐free second‐order Douglas–Kroll transformed Hamiltonian of Hess.