Complex absorbing potential equation-of-motion coupled-cluster method yields smooth and internally consistent potential energy surfaces and lifetimes for molecular resonances

T.-C. Jagau and A.I. Krylov
J. Phys. Chem. Lett. 5, 3078 – 3085 (2014)

The recently developed equation-of-motion electron-attachment coupled-cluster singles and doubles (EOM-EA-CCSD) method augmented by a complex-absorbing potential (CAP) is applied to the 2Pig resonance of N₂^- and the 2Sigmau+ resonance of H₂^- at various internuclear distances. The results illustrate the advantages of EOM-CC for treating resonance states over state-specific approaches. CAP-EOM-EA-CCSD produces smoothly varying potential energy curves and lifetimes, both for Sigma and Pi resonances. The computed lifetimes and energy differences between the neutral and electron-attached states are internally consistent, i.e., the resonance width becomes zero at the same internuclear distance where the energy of the electron-attached state drops below that of the neutral one. Such a smooth and internally consistent behavior is only achieved when the perturbation due to the CAP is removed using a first-order de-perturbative correction [J. Phys. Chem. Lett. 5, 310 (2014)]; the evaluation of resonance positions and widths from raw (uncorrected) energies leads to unphysical discontinuities and does not correctly describe the conversion of a resonance to a bound state at large internuclear distances.

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