Spin-Conserved and Spin-Flip Optical Excitations from the Bethe–Salpeter Equation Formalism

Like adiabatic time-dependent density-functional theory (TD-DFT), the Bethe–Salpeter equation (BSE) formalism of many-body perturbation theory, in its static approximation, is “blind” to double (and higher) excitations, which are ubiquitous, for example, conjugated molecules like polyenes. Here, we apply spin-flip ansatz (which considers lowest triplet state as reference configuration instead singlet ground state) BSE order access, particular, excitations. The present scheme based on a spin-unrestricted version GW approximation employed compute charged excitations and screened Coulomb potential required calculations. Dynamical corrections optical taken into account via an unrestricted generalization our recently developed (renormalized) perturbative treatment. performance illustrated by computing excited-state energies beryllium atom, hydrogen molecule at various bond lengths, cyclobutadiene rectangular square-planar geometries.