On the energy difference between the l P and 3 P states of the beryllium isoelectronic sequence

Wave functions of the 1 S (ground state), 3 P and 1 P states for the beryllium isoelectronic sequence have been obtained in various approximations. The HF 2 p orbitals for the 1 P and 3 P states are similar except for Be, where the 2 p orbital is quite diffuse for the 1 P state. The difference between the experimental E ( 1 P ) – E ( 3 P ) and the HF E ( 1 P ) – E ( 3 P ) is 0.62 eV for Be and 1.17 ∼ 1.40 eV for B + ∼ F 5+ . The disagreements are attributed to the correlation effects between the 2 s and 2 p electrons. This is confirmed by ci calculations. It is shown that a limited basis SCF calculation reproduces the above feature of the HF results if we treat the orbital exponents as the variational parameters. The use of the Slater values for the orbital exponents is shown to be inadequate especially for the Be 1 P state. The conclusions of this paper will be useful for discussing the V–T separations of H 2 and C 2 H 4 .