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Equation-of-motion Calculations With Continuum Orbitals

Equation-of-motion coupled cluster (EOM-CC) calculations can be performed with one or more "continuum" orbitals while using the standard EOMEE excitation energy code. Exciting into an unoccupied continuum orbital gives the same result as an ionization potential calculation (EOMIP), while exciting from an occupied continuum orbital gives the electron affinity (EOMEA). Multiple electrons can be excited to or from continuum orbitals to give EOMDEA, EOMDIP, etc. The keywords to request one or more continuum orbitals be added are

CONTINUUM=OCCUPIED
CONTINUUM=DOCCUPIED
CONTINUUM=VIRTUAL
CONTINUUM=DVIRTUAL

Options OCCUPIED and DOCCUPIED add one or two continuum orbitals which will be occupied in the SCF calculation, and which will be placed in the HOMO and HOMO-1 positions. Options VIRTUAL and DVIRTUAL add one or two continuum orbitals which will be unoccupied in the SCF calculation, and which will be placed in the LUMO and LUMO+1 positions. For occupied virtual orbitals, the molecular charge should be adjusted correspondingly for the 'nonexistent' continuum electrons. With these options, the %excite* input form for the EOM code should be used to select the proper EOM sector (EOMIP, EOMDIP, etc.). Use of continuum orbitals in not necessarily restricted to the EOMEE codes (VEE and SDCC), and could theoretically be used with VIP and VEA.