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(June 30, 2021)

For the description of metastable electronic states and the calculation of
positions and widths of such electronic resonances, the *complex absorbing potential* (CAP)
methodology
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940
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J. Phys. B

(1993),
26,
pp. 4503.
Link
has been combined with all available non-CVS ADC methods
using a subspace projection approach.
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Int. J. Quantum Chem.

(2001),
82,
pp. 218–226.
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. 958 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,
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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 7.10.8.