AtomPert

AtomPert is a theoretical framework and software methodology for analyzing perturbations in atomic systems. It provides a structured approach to compute how external influences alter energy levels, wavefunctions, and observable properties of atoms and small molecules.

The framework treats the total Hamiltonian as H = H0 + V, where H0 is the unperturbed Hamiltonian and V represents perturbations such as electric or magnetic fields, spin–orbit couplings, or environmental interactions. Calculations proceed using perturbation theory to obtain energy and state corrections; both Rayleigh-Schrödinger and Brillouin-Wigner formulations are supported. AtomPert is designed to be compatible with established electronic-structure methods to furnish accurate H0 and unperturbed wavefunctions.

Key features include modular operator algebra for V, support for static and dynamic perturbations, and interfaces for computing properties such as polarizabilities, hyperpolarizabilities, Stark and Zeeman shifts, and transition amplitudes. The framework emphasizes reproducibility, with transparent accounting of approximations and numerical parameters.

Applications span atomic spectroscopy, precision metrology, sensor design, and the analysis of perturbative effects in quantum information platforms. It also serves as a teaching tool for illustrating how small perturbations propagate through quantum systems.

Limitations include the breakdown of perturbation theory for strong perturbations or near-degeneracies, dependence on the choice of H0 and basis sets, and computational cost for high-order corrections or large systems. AtomPert is typically available as an open-source library or plugin that can be integrated with common quantum-chemical packages, with APIs in Python and C++. See also: perturbation theory, Stark effect, polarizability.