From 987143abc37941a4ea0e31bbc7cb028c278c9de3 Mon Sep 17 00:00:00 2001
From: eschnett <eschnett@578cdeb0-5ea1-4b81-8215-5a3b8777ee0b>
Date: Mon, 29 Oct 2012 22:38:34 +0000
Subject: Slightly reduce round-off error

git-svn-id: http://svn.cactuscode.org/arrangements/CactusNumerical/MoL/trunk@182 578cdeb0-5ea1-4b81-8215-5a3b8777ee0b
---
 src/RK4.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/RK4.c b/src/RK4.c
index f0ebc8f..1d790ab 100644
--- a/src/RK4.c
+++ b/src/RK4.c
@@ -118,11 +118,11 @@ CCTK_WARN(0, "not implemented");
   {
     case 0:
       alpha = 1.0 / 3.0;
-      beta  = 0.5;
+      beta  = 1.0 / 2.0;
       break;
     case 1:
       alpha = 2.0 / 3.0;
-      beta  = 0.5;
+      beta  = 1.0 / 2.0;
       break;
     case 2:
       alpha = 1.0 / 3.0;
@@ -133,6 +133,7 @@ CCTK_WARN(0, "not implemented");
       beta  = 1.0 / 6.0;
   }
 
+  /* Initialise alpha before the first intermediate step */
   if (MoL_Intermediate_Steps == (*MoL_Intermediate_Step))
   {
     sum_alpha = 0.0;
@@ -292,7 +293,7 @@ CCTK_WARN(0, "not done");
 #pragma omp parallel for
       for (index = 0; index < arraytotalsize; index++)
       {
-        UpdateVar[index] += ScratchVar[index] - 4.0 / 3.0 * OldVar[index];
+        UpdateVar[index] += ScratchVar[index] - (sum_alpha - 1.0) * OldVar[index];
       }
     }
     arrayscratchlocation += arraytotalsize;
-- 
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