Parameter RCUT
In a typical scattering calculation, a large channel basis is needed only
at small values of Jtot. As Jtot increases, the centrifugal barrier prevents the collision partners from penetrating into the strong coupling
region, so that the effective distortion of the asymptotic Hamiltonian is much
less. Considerable time can be saved by dropping channels as Jtot
increases.
This is done in the HibridonTM code by means of the parameter RCUT, as
follows:
In the subroutine basis, the centrifugal barrier in each channel,
evaluated at R = RCUT,
h2 li (li + 1) / (2 µ RCUT2)
where li is the orbital angular momentum of the ith channel, is compared
to the internal energy of the channel,
i. If the centrifugal barrier at
R = RCUT is greater than
i, then the channel is still closed at R = RCUT. If
any of the channels which are open asymptotically are found to be closed
at R = RCUT, then these channels as well as all other closed channels are
dropped from the channel basis.
We have found this to be an effective and simple way of dropping channels
at large Jtot. The rapidity by which the channels are dropped is
controlled by the magnitude of RCUT. Obviously, if RCUT is taken equal to
RENDAI - the ending point of the integration - then no channels will ever
be dropped. A good rule of thumb is to chose initially a small value for
RCUT in the range RENDAI/3 to RENDAI/2 and then increase RCUT only if
necessary.
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