Index: kernel/src/optimizers/movers/BallMover.cpp
===================================================================
--- kernel/src/optimizers/movers/BallMover.cpp	(revision 388)
+++ kernel/src/optimizers/movers/BallMover.cpp	(working copy)
@@ -54,7 +54,7 @@
   for (unsigned int i = 0; i < vars.size(); ++i) {
     add_key(vars[i]);
   }
-  max_step_ = max;
+  set_radius(max);
 }
 
 void BallMover::generate_move(float scale)
@@ -64,7 +64,7 @@
     for (unsigned int j = 0; j < number_of_float_keys(); ++j) {
       center[j] = get_float(i, j);
     }
-    std::vector<Float> npos = random_point_in_sphere(center, scale * max_step_);
+    std::vector<Float> npos = random_point_in_sphere(center, scale * radius_);
     for (unsigned int j = 0; j < number_of_float_keys(); ++j) {
       propose_value(i, j, npos[j]);
     }
Index: kernel/src/optimizers/movers/NormalMover.cpp
===================================================================
--- kernel/src/optimizers/movers/NormalMover.cpp	(revision 388)
+++ kernel/src/optimizers/movers/NormalMover.cpp	(working copy)
@@ -17,12 +17,11 @@
                          const FloatKeys &vars,
                          Float max)
 {
-  IMP_assert(max != 0, "Must have some width");
   add_particles(pis);
   for (unsigned int i = 0; i < vars.size(); ++i) {
     add_key(vars[i]);
   }
-  stddev_ = max;
+  set_sigma(max);
 }
 
 void NormalMover::generate_move(float scale)
Index: kernel/include/IMP/optimizers/movers/BallMover.h
===================================================================
--- kernel/include/IMP/optimizers/movers/BallMover.h	(revision 388)
+++ kernel/include/IMP/optimizers/movers/BallMover.h	(working copy)
@@ -1,5 +1,6 @@
 /**
- *  \file BallMover.h    \brief A modifier which perturbs a discrete variable.
+ *  \file BallMover.h
+ *  \brief A modifier which variables within a ball.
  *
  *  Copyright 2007-8 Sali Lab. All rights reserved.
  *
@@ -23,13 +24,30 @@
 class IMPDLLEXPORT BallMover :public MoverBase
 {
 public:
+  /** The attributes are perturbed within a pall whose dimensionality is 
+      given by the number of attributes and radius by the given value.
+     \param[in] ps The particles to perturb.
+     \param[in] vars The variables to use (normally the keys for x,y,z)
+     \param[in] radius The radius deviation to use.
+   */
   BallMover(const Particles &pis, const FloatKeys &vars,
-            Float max);
+            Float radius);
+  //!
+  void set_radius(Float radius) {
+    IMP_check(radius > 0, "The radius must be positive",
+              ValueException("Negative radius"));
+    radius_=radius;
+  }
+  //!
+  Float get_radius() const {
+    return radius_;
+  }
 protected:
+  /** \internal */
   void generate_move(float a);
 
 private:
-  Float max_step_;
+  Float radius_;
 };
 
 } // namespace IMP
Index: kernel/include/IMP/optimizers/movers/NormalMover.h
===================================================================
--- kernel/include/IMP/optimizers/movers/NormalMover.h	(revision 388)
+++ kernel/include/IMP/optimizers/movers/NormalMover.h	(working copy)
@@ -1,6 +1,6 @@
 /**
  *  \file NormalMover.h    
- *  \brief A modifier which perturbs a point with a gaussian.
+ *  \brief A modifier which perturbs a point with a normal distribution.
  *
  *  Copyright 2007-8 Sali Lab. All rights reserved.
  *
@@ -15,22 +15,34 @@
 namespace IMP
 {
 
-//! Modify a set of continuous variables.
-/** The variables are perturbed within a ball of the 
-    given radius.
-    \ingroup mover
+//! Modify a set of continuous variables using a normal distribution.
+/** \ingroup mover
  */
 class IMPDLLEXPORT NormalMover :public MoverBase
 {
 public:
-  NormalMover(const Particles &pis, const FloatKeys &vars,
-              Float stdev);
+  /**
+     \param[in] ps The particles to perturb.
+     \param[in] vars The variables to use (normally the keys for x,y,z)
+     \param[in] sigma The standard deviation to use.
+   */
+  NormalMover(const Particles &ps, const FloatKeys &vars,
+              Float sigma);
   void set_particles(const Particles &ps) {
     MoverBase::clear_particles();
     MoverBase::add_particles(ps);
   }
+  void set_sigma(Float sigma) {
+    IMP_check(sigma > 0, "Sigma must be positive",
+              ValueException("Negative sigma"));
+    stddev_=sigma;
+  }
+  Float get_sigma() const {
+    return stddev_;
+  }
 protected:
-   virtual void generate_move(float f);
+  /** \internal */
+  virtual void generate_move(float f);
 private:
   Float stddev_;
 };
Index: kernel/include/IMP/optimizers/MoverBase.h
===================================================================
--- kernel/include/IMP/optimizers/MoverBase.h	(revision 388)
+++ kernel/include/IMP/optimizers/MoverBase.h	(working copy)
@@ -60,7 +60,7 @@
   }
   Particle *get_particle(unsigned int i) const {
     IMP_check(i< ps_.size(), "Bad index in MoverBase::get_particle",
-              IndexException());
+              IndexException("Particle index out of range in mover base"));
     return ps_[i];
   }
 
@@ -114,7 +114,9 @@
       \param[in] t The value to propose
    */
   void propose_value(unsigned int i, unsigned int j, Float t) {
-    get_particle(i)->set_value(fkeys_[j], t);
+    if (get_particle(i)->get_is_optimized(fkeys_[j])) {
+      get_particle(i)->set_value(fkeys_[j], t);
+    }
   }
   //! Propose a value
   /** \param[in] i The index of the particle.