For the description of metastable electronic states and the calculation of
positions and widths of such electronic resonances, the complex absorbing potential (CAP)
J. Phys. B
(1993), 26, pp. 4503. has been combined with all available non-CVS ADC methods using a subspace projection approach. 1057 Int. J. Quantum Chem.
(2001), 82, pp. 218–226. , 259 J. Chem. Phys.
(2021), 155, pp. 054103.
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.
(2002), 368, pp. 1. 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 (or CAP_ETA = 100000). Different CAP types
can be employed, however, it is generally recommended to use a smoothed Voronoi
J. Chem. Theory Comput.
(2015), 11, pp. 4627. 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.