The Bethe-Salpeter QED wave equation for bound-state computations of atoms and molecules

Interactions in atomic and molecular systems are dominated by electromagnetic forces and the theoretical framework must be in the quantum regime. The physical theory for the combination of quantum mechanics and electromagnetism, quantum electrodynamics has been established by the mid-twentieth century, primarily as a scattering theory. To describe atoms and molecules, it is important to consider bound states. In the non-relativistic quantum mechanics framework, bound states can be efficiently computed using robust and general methodologies with systematic approximations developed for solving wave equations. With the sight of the development of a computational quantum electrodynamics framework for atomic and molecular matter, the field theoretic Bethe-Salpeter wave equation expressed in space-time coordinates, its exact equal-time variant and emergence of a relativistic wave equation is reviewed. A computational framework, with initial applications and future challenges in relation with precision spectroscopy, is also highlighted

Multiple bond breaking with APSG based correlation methods -- Comparison of two approaches

Antisymmetrized product of strongly orthogonal geminals (APSG) Ansatz is a computationally economic wavefunction class with favourable formal properties. These include extensivity, variational determination of the wavefunction parameters or qualitatively correct description of single bond dissociation. Breaking multiple bonds or non-isolated single bonds results in fragments of incorrect spin state when computed by APSG. This has been identified as a potential problem in APSG based linearized coupled-cluster approach (LCC). An alternative correction scheme based on the extended random phase approximation (ERPA) is investigated from this point of view, in parallel with LCC. The two methods are compared formally. Potential energy curves and atomic spin by APSG based LCC and ERPA are presented on illustrative examples for multiple bond breaking. Origin of the marked difference between the behaviour of LCC and ERPA is explored