[erlang-questions] erlang float comparison

Richard O'Keefe ok@REDACTED
Tue May 15 04:45:29 CEST 2012

The original message said something about comparing dihedral angles in
molecules.  I find myself wondering what the actual problem is that
the OP is trying to solve, and how much difference between two angles
is *physically* reasonable.

In one document I just found on the web, the H-O-H bond angle is
given as 107.5 degrees in one place and 104.5 degrees in another.
Another document calls it 105 degrees.

My 1st-year chemistry days are long past, but web pages I've checked
today seem not to quote any bond angles to better than 1 part in 1000
precision.  And since angles are constrained to [0,one circle), it
looks to me very much as if we have a case for representing bond angles
and dihedral angles as *fixed point* numbers, or at any rate for
rounding them to multiples of say (one circle)/65536 and comparing
the angles as integers.

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