Free-particle Green's function matrix elements over spherical Gaussian and plane-wave-modulated Gaussian basis...

Free-particle Green's function plays a central role in the theoretical description of electron scattering and autoionization processes in quantum physics and chemistry. Recently, Gaussian basis set approaches have become increasingly important in applications to unbound and metastable electronic states. However, the practical use of such methods has been limited by the lack of efficient and compact analytical expressions for matrix elements of the free-particle Green's function in Gaussian-based representations. Here we present a direct, general, and numerically stable analytical formalism for evaluating one- and two-center matrix elements of the free-particle Green's operator over spherical Gaussian basis functions and plane-wave-modulated spherical Gaussians. The derivation explicitly exploits the rotational symmetry of the Green's operator together with the addition theorem of harmonic polynomials, leading to compact closed-form expressions and efficient recurrence relations. We also analyze the asymptotic behavior of the free-particle Green's function matrix elements, which is essential in the description of low-energy continuum electrons using finite Gaussian basis sets.