Resonances are electronically unbound states; they are metastable and decay by ejecting an electron.

Resonances are embedded in the continuum and are not accessible by standard quantum-chemistry methods.

Within non-Hermitian quantum mechanics, resonances become \(L^2\)-integrable, discrete states with complex energies (the real part describes resonance position and the imaginary part describes the resonance width); this enables calculation of resonances by standard quantum chemistry methods.

Non-Hermitian extensions of quantum chemistry include 1) scaling all coordinates of the Hamiltonian with a complex factor exp(i*\(\theta\)) (the complex-scaling method); 2) using complex basis functions; 3) augmenting the Hamiltonian by a local complex absorbing potential i*\(\eta\)*W(r).

EOM-CC methods provide an excellent platform for treating resonances because they efficiently include correlation and tackle multiple electronic states of different character in a balanced fashion.

Energies, properties, and nuclear gradients are available for several complex-valued EOM-CC methods.