Relativistic effects on the ground state of positronic lithium

Positronic lithium, the first theoretically predicted positronic atom, has yet to be observed experimentally due to its weakly bound nature and short lifetime. Consequently, studies of its structural and annihilation properties can provide valuable guidance for future experimental efforts. In this work, we investigate the relativistic effects on positron binding to atomic lithium by evaluating the expectation values of relativistic operators. Our calculations show that while the total relativistic correction of positronic lithium is comparable in magnitude to the binding energy, the corrections for the bound and dissociated states are nearly identical and largely cancel each other, resulting in only a minor effect on the binding energy. Specifically, the inclusion of relativistic corrections alters the binding energy of positronic lithium and its isotopes by less than 0.13% for spin-triplet states and 0.04% for spin-singlet states. We also calculate the annihilation rates and lifetimes for both spin-triplet and spin-singlet states of positronic lithium.