Confined variational calculation of low-energy Ps-${\mathrm{Li}}^{+}$ scattering

A theoretical investigation is conducted on the $s$- and $p$-wave elastic scatterings of positronium by a lithium ion ${\text{Li}}^{+}$ with the scattering energy below 1.41 eV, corresponding to the threshold of the $e^{+}$-Li channel. The confined variational method is applied to serve as the theoretical framework for this study. To accurately account for correlations between involved particles, explicitly correlated Gaussians are employed as basis functions, which are optimized through a hybrid approach combining stochastic variational and energy-gradient-based methods. Additionally, a straightforward yet effective algorithm is developed for the automatic adjustment of confining potentials. The $s$-wave zero-energy pickoff annihilation parameter ${}^1{Z}_{\text{eff},0}$ is accurately determined to be $0.126\pm 0.002$, which yields an enhancement factor of 1.88 compared with the value 0.067 obtained using the fixed-core stochastic variational method [Phys. Rev. A 65, 034709 (2002)]. Finally, a broad $p$-wave resonance structure is predicted at the incident energy of approximately 0.27 eV, with the annihilation parameter ${}^1{Z}_{\text{eff},1}$ at the resonance center estimated to be around 0.034.