Some restrictions on the use of reduced density matrices in atomic calculations

The method of reduced density matrices for obtaining the ground-state energy of an atomic system is developed, making full use of the symmetry relations for orbital and spin angular momentum. These, together with an extensive set of Hamiltonian-dependent identities, serve to decrease the number of parameters which must be varied in the density-matrix variational principle. With only a small number of parameters required, inequalities such as the Pauli restriction can then be enforced. Numerical calculations for C++ show the relative ineffectiveness of certain low-lying geminals (in the Γ(2) expansion) in reaching the Pauli restriction limit, and thus point the way to significant improvement.