[gmx-developers] Re: Cubic splines - again

Erik Lindahl lindahl at cbr.su.se
Tue Aug 22 15:00:01 CEST 2006


On Aug 22, 2006, at 12:46 PM, Berk Hess wrote:

> But I think we would still prefer to have continuous second  
> derivatives as well.
> So the problem is then not in the interloops, but in the generation  
> of the tables.
> There we should not use the second derivative of the potential,
> but rather the second derivative of the cubic spline fit of the  
> potential.

Agreed. In principle, the current innerloop code will work unmodified  
for any third-power splines, including e.g. akima.

Further, on most architectures the costly step is to (a) calculate a  
table index (truncation, etc.) and (b) fetch the table data from  
memory. The amount of floating-point calculations involved is not  
critical (though not entirely unimportant).

If we come up with a better table implementation this part is _very_  
easy to change, including the assembly implementations. Not quite  
search-and-replace, but almost.



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