From 4e599fdf8090143ac787e179c8dd52d305ded1a2 Mon Sep 17 00:00:00 2001
From: jbphet <john.blanco@colorado.edu>
Date: Wed, 1 Aug 2018 12:09:04 -0600
Subject: [PATCH] added code to correct for potential drift in the monatomic
 solid "crystal", see
 https://github.com/phetsims/states-of-matter-basics/issues/15

---
 .../model/engine/AbstractPhaseStateChanger.js     | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/js/common/model/engine/AbstractPhaseStateChanger.js b/js/common/model/engine/AbstractPhaseStateChanger.js
index cf38a2b2..a91e383f 100644
--- a/js/common/model/engine/AbstractPhaseStateChanger.js
+++ b/js/common/model/engine/AbstractPhaseStateChanger.js
@@ -27,9 +27,12 @@ define( function( require ) {
    * @constructor
    */
   function AbstractPhaseStateChanger( multipleParticleModel ) {
+
+    // @private
     this.multipleParticleModel = multipleParticleModel;
     this.moleculeLocation = new Vector2();
     this.random = phet.joist.random;
+    this.reusableVector = new Vector2();
   }
 
   statesOfMatter.register( 'AbstractPhaseStateChanger', AbstractPhaseStateChanger );
@@ -128,6 +131,7 @@ define( function( require ) {
       var moleculesPlaced = 0;
       var xPos;
       var yPos;
+      this.reusableVector.setXY( 0, 0 );
       for ( var i = 0; i < numberOfMolecules; i++ ) {
 
         // Position one layer of molecules.
@@ -149,6 +153,9 @@ define( function( require ) {
           var yVel = temperatureSqrt * this.random.nextGaussian();
           moleculeVelocities[ moleculeIndex ].setXY( xVel, yVel );
 
+          // Track total velocity in the X direction.
+          this.reusableVector.addXY( xVel, yVel );
+
           // Assign an initial rotational angle (has no effect for single-atom data sets)
           moleculeRotationAngles[ moleculeIndex ] = randomizeRotationalAngle ?
                                                     this.random.nextDouble() * 2 * Math.PI :
@@ -158,6 +165,14 @@ define( function( require ) {
           moleculesInsideContainer[ i ] = true;
         }
       }
+
+      // The number of particles is often not large enough to count on having a zero average X and Y velocity, so the
+      // substance can end up with some initial drive.  This compensates for that problem.
+      var xAdjustment = -this.reusableVector.x / numberOfMolecules;
+      var yAdjustment = -this.reusableVector.y / numberOfMolecules;
+      for ( i = 0; i < numberOfMolecules; i++ ) {
+        moleculeVelocities[ i ].addXY( xAdjustment, yAdjustment );
+      }
     },
 
     /**