Treating unbound states¶
Identifying unbound states¶
Unbound states appear above the state dissociations. The Duo is developed to treat bound state problems with an effective boundary condition for the rovibronic eigenfunctions to vanish at the borders of the simulation grid. However it is a typical problem when an electronic system contans unbound region between states or above their dissociations, where unboud solutions are possible. Moreover, some of these unbound eigenfunctions are exactly zero at (other side is automatically zero duo to the steep repulsive wall). Duo methodology can be applied also to compute the unbound spectra, here we show how to remove the spurious unbound states from the spectra (line lists) calculations and thus to produce bound a line list. The unbound wavefunctions can be identified based on their asymptotic properties via non-zero density in the small region of at the right border :
is a small threshold value. The default threshold value is is chosen as . The threshold value can be specified in the input file using the keyword thresh_bound.
The default value of is 0.5 , which can be changed using the keyword THRESH_DELTA_R
(also in ).
Excluding unbound states¶
Once the unbound states are identified they can be excluded from the intensity or line list calculations using the bound keyword in the INTENSITY section, which tells Duo to compute bound-bound spectra only, e.g.:
Example:
intensity
absorption
bound
thresh_intes 1e-50
thresh_bound 1e-6
thresh_delta_R 1 (Angstrom)
temperature 3000 (K)
nspin 2.5 1.5
linelist AlCl-37_61_J160
J 0, 20
freq-window 0.0 48000.0
energy low 0.0 30000, upper 0.0,48000
end
Her, the integrated density of the given state over the region of will be compared to the threshold value of . The state in question is considered as unbound if .
This selection is very dependent on the geometry of the box and the integrated region and thus will be different for different systems. The selection criterium for the quasi-bound states is not very well defined and must be chosen on the case-by-case basis. For example, it is sometimes useful to plot the corresponding reduced densities, check the lifetimes or even compare to the experiment to see what should be considered as ‘bound’ states.
An alternative way, less dependent on the specific geometry is to specify the threshold using the average density over the specific integration region:
defined using the THRESH_AVERAGE_DENSITY
keyword, for example:
- INTENSITY
absorption bound THRESH_BOUND 0.1 THRESH_DELTA_R 1 (Angstrom) THRESH_AVERAGE_density 1e-4 THRESH_DELTA_R 4 THRESH_INTES 1e-40 THRESH_LINE 1e-40 thresh_dipole 1e-7 TEMPERATURE 750 linelist AlH_446_A-X_L60.695_J10 J, 0.0, 1 freq-window 0.0, 30000.0 energy low -0.001, 30000.00, upper -0.00, 30000.0
END
The default value of is .
Excluding bound upper states¶
Sometimes only the transitions to the unbound state are needed. In this case we exclude tansitions to the upper bound states with a keyword unbound placed anywhere in the INTENSITY section.
Example:
intensity
absorption
unbound
thresh_intes 1e-50
thresh_bound 1e-6
temperature 3000 (K)
nspin 2.5 1.5
linelist AlCl-37_61_J160
J 0, 20
freq-window 0.0 48000.0
energy low 0.0 30000, upper 0.0,48000
end