[Ifeffit] Hard restraints?

[Matt Newville]

Hi Norbert, Scott,

On Wed, 9 Feb 2005, Norbert Weiher wrote:
 
> towards the "bound (min,max)" issue:
> 
> I think that is what many people dream of in IFEFFIT. By now,
> the implementation of a restraint is (as Matt's example
> showed) a bit scary. And we all have seen the tricks of fits
> like negative DW factors, silly CNs and so on. So a simple
> "bound" command as mentioned would for sure make - especially
> for new users - life a bit easier.

Just for completeness, there are two different ways to give
bounds to variables. The first uses max/min:
   guess x_var = 1
   def   x     = max(low, min(high, x_var))

which gives infinitely hard walls: x will never go outside the
bounds.  

The second approach is to calculate a penalty when the value
goes out of bounds.  My earlier post gave an ugly way to do
this, and a built-in 'bound()' function could also do this:
   penalty = bound(x_var, low, high, scale)

x_var may go outside the limits, but with a penalty of
   penalty =   (x_var-high)*scale  for x_var > high 
   penalty =  -(x_var-low)*scale   for x_var < low

A very large value for scale would make a very hard wall, but 
the strength of the wall is tweakable.  This penalty can then be 
used as a restraint.

A bound() function would be easy enough to implement, and I'll
do this for the next vesion (but see below).

Possibly related to this, Bruce has suggested a couple of times
that restraints should be implemented with a command restraint()
similar to set() or def() rather than an additional scaler
parameter to feffit().  That might fit in with this....

--Matt

On upcoming versions:

The source code for next version of Ifeffit won't be for several
weeks (I won't start working on it for at least two weeks).  No
estimate on a Windows dll.  Did I mention that releases of
Windows binaries and dlls will lag significantly?  I did ask for
feedback about a recent windows update and got exactly two
responses.  I also asked for help building windows executables
and heard no replies.  I take this to mean that there is little
interest in the Windows versions of these programs.  I cannot
make Windows executables for Athena, Artemis, etc even if I
wanted too, and don't expect that Bruce will be making them real
soon either.  That's all to say that if you're expecting a
windows version with these changes, you'll be waiting a while.