From apodtele@kgi.edu Wed Dec  7 11:53:10 2005
Date: Thu, 4 Aug 2005 22:42:48 -0700
From: Alexei Podtelezhnikov <apodtele@kgi.edu>
To: ccp4@ccp4.ac.uk
Subject: [ccp4]: tiny impovement to dihedral algorithm

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Hi,

This small patch brings ccp4's dihedral angle code even closer to perfection:

--- geomcalc.f  2005-08-04 22:26:17.000000000 -0700
+++ geomcalc.f  2005-08-04 22:31:12.000000000 -0700
@@ -1494,9 +1494,7 @@
       call vdif(v3, x4, x3)
       call cross(q, v1,v2)
       call cross(t, v2, v3)
-      call cross(v1, q, t)
-      tangle = atan2(sqrt(dot(v1,v1)), dot(q,t))*rtod
-      if (dot(v1,v2) .lt. 0.0) tangle = -tangle
+      tangle = atan2(dot(q,v3)*sqrt(dot(v2,v2)), dot(q,t))*rtod
       return
       end
 c

So we can drop if-check and a cross-product in favor of a dot-product, atan2 does the rest of the job. The patch is also attached.

Alexei

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