Resonance energies from complex absorbing potentials: a complex basis function approach in the η → 0⁺ limit

Complex absorbing potentials (CAPs) are non-physical in nature, and they introduce artificial perturbations to the system, resulting in unwanted shifts in the calculated resonance energies. This study presents a novel methodology for eliminating these artificial reflections using complex basis functions (CBFs). The eigenfunctions obtained at high CAP strength parameter (η) are used as CBFs, and resonance energies are calculated in the limit of η → 0⁺. This method not only generates perturbation-free resonance energies, but also produces the resonance wavefunction at η → 0⁺. The resulting complex resonance energies obtained using these CBFs are independent of both the CAP box-size parameters and the optimal value of η. Consequently, the proposed approach eliminates the need to optimise the CAP-onset parameters while providing reliable and parameter-independent resonance energies.