7.11.8 CAP/ADC Methods for the Description of Metastable Electronic States

For the description of metastable electronic states and the calculation of positions and widths of such electronic resonances, the complex absorbing potential (CAP) methodology ¹⁰⁹⁸ Riss U. V., Meyer H.-D.
J. Phys. B: At. Mol. Opt. Phys.
(1993), 26, pp. 4503. Link has been combined with all available non-CVS ADC methods using a subspace projection approach. ¹¹⁹⁷ Sommerfeld T., Santra R.
Int. J. Quantum Chem.
(2001), 82, pp. 218–226. Link ^{, 295} Dempwolff A. L. et al.
J. Chem. Phys.
(2021), 155, pp. 054103. Link

In this approach, the CAP is projected onto the subspace spanned by a number of converged (ADC, IP-ADC or EA-ADC) states. For this purpose one-electron state and state-to-state transition densities computed using the second-order ISR are exploited.

The generation of CAP trajectories and determination of the resonance parameters can be done a posteriori (see Ref.  1119 Santra R., Cederbaum L. S.
Phys. Rep.
(2002), 368, pp. 1. Link for details), i.e., only a single electronic structure calculation has to be performed. As a distinct feature of this approach, a series of different CAP onsets can be handled in a single ADC calculation.

CAP/ADC calculations are invoked by setting ADC_CAP = 1, automatically implying a CAP strength of $\eta =1$ (or CAP_ETA = 100000). Different CAP types can be employed, however, it is generally recommended to use a smoothed Voronoi CAP, ¹¹⁹⁶ Sommerfeld T., Ehara M.
J. Chem. Theory Comput.
(2015), 11, pp. 4627. Link which is requested by setting CAP_TYPE = 2. For this CAP type, a series of different onsets can be controlled using the CAP_X, CAP_X_STEP and CAP_X_END keywords. For example, subspace-projected CAP/ADC output for onset values of 2.0, 3.0 and 4.0 a.u. can be obtained by setting CAP_X = 2000, CAP_X_STEP = 1000 and CAP_X_END = 4000.

For further details on different CAP types and their control, also see Section 7.10.9.