Calculations of 2s²S_1/2-2p²P_1/2,3/2 transition energies for lithium-like systems from Na IX to Ca XVIII

The transition energies (2s²S_1/2 - 2p²P_1/2,3/2) of lithium-like systems with nuclear charge from Z = 11-20 are calculated by using a full-core plus correlation method. Relativistic and mass-polarization effects on the energies are included as the first-order perturbation corrections. The quantum-electrodynamics contributions to the transition energy are evaluated by using effective nuclear charge. Our results are in excellent agreement with previous theoretical and experimental data available in the literature.