Electronic Wavefunctions for Atoms. III. Partition of Degenerate Spaces and Ground State of C

The atomic configuration‐interaction (CI) expansion is expressed in terms of constituents which are invariant under linear transformations of the radial one‐electron basis. For open‐shell states, the degenerate $\mathrm{LS}$ spaces are partitioned into maximal Hartree–Fock (HF) noninteracting spaces and minimal HF interacting spaces. This particular partition is shown to improve considerably the convergence of the atomic CI expansion, and it may be applied to molecular CI expansions in terms of symmetry orbitals. An exhaustive calculation of the ground state of C is carried out. A well‐defined linear transformation of the radial one‐electron basis is made in order to improve further the convergence of the CI series. The final 234‐term CI expansion (formed from a determinantal space of dimension 4836) obtained with an optimized $\text{7s6p4d3f}$ basis, gives an energy $\text{E\hspace{0.17em}=\hspace{0.17em}−37.83378}\mathrm{a.u.}$ (C), accounting for more than 93% of the correlation energy. The C wavefunction is believed to be expressed in its most rapidly convergent form. The advantages of the present approach to atomic CI calculations are discussed.