Calculating Unbound Resonances with Complex-Valued EOM-CC Methods

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  • Resonances are metastable unbound states with complex energies, of which the real parts denote the energies of the states and the imaginary parts represent lifetime. They are beyond the reach of common quantum chemistry methods.
  • Resonances can be accessed by common quantum chemistry methods with the tricks 1) scaling all coordinates of the Hamiltonian with a complex factor exp(i*𝜃) (the complex scaling (CS); method); 2) augmenting the Hamiltonian by a artificial local complex absorbing potential (CAP) i*𝜂*W(r). In both methods the resonance state is found when the complex scaling rotation angle 𝜃 or the CAP intensity 𝜂 stabilize the complex resonance energy.
  • EOM-CC methods are the method of choice for resonances because they can efficiently include correlation for highly excited states. Q-Chem offers calculation options for both CS and CAP EOM-CC methods.

Stabilization of the the 2𝚷g resonance of N2− with increasing CAP intensity from EOM-EA-CCSD calculations

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Potential energy curve for the 2𝚷 resonance of CuF− from CAP-EOM-CCSD calculations

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