Re: [Wavepacket-user] questions about the wavepacket

[Ulf L. <ulf...@de...>]

On Wed, 2010-10-20 at 10:35 +0800, wanglei365x wrote:
> hello,
> i am one of the wavepacket-users, i am confusing about the following
> questions: 

Great, feedback. :)

> what does the variable 'hamilt.truncate' mean, why we need it, and how
> to decide it?

The variable used to be more central, and is admittedly hard to follow
in the code flow. However, it still has two uses:

1. Less important: When plotting, in the absence of other parameters, it
is used to select some energy ranges for the plot. However, there are
other variables nowadays, which are not really documented and sometimes
do the same (making developing note).

2. For the Chebychev propagator to converge, all eigenvalues of the
Hamiltonian must be in a finite range [min,max]. Furthermore, the
propagator is faster (allows larger time steps with the same accuracy
and cost) the smaller the range delta = max-min is. In this case,
hamilt.truncate.{min|max} is used to truncate the potential and kinetic
energy grids so that they are guaranteed to have eigenvalues in the
range [min,max] (potentials) or [0,delta] (kinetic energy). Then the
bounds are added to give a worst case estimate of the smallest and
largest eigenvalue of the total Hamiltonian (stored in hamilt.{min|
max}), which you can override if you know better.

If you normally use the split operator and do not care too much about
the plots (or fiddle around with them using plots.pot.{min|max}) then
you can ignore this variable. If you set it, set it such that the
important part of the potential and kinetic energy is safely within the
boundaries.


Ulf