diff --git include/openbabel/atom.h include/openbabel/atom.h
index 74553914..0f35922e 100644
--- include/openbabel/atom.h
+++ include/openbabel/atom.h
@@ -284,7 +284,7 @@ namespace OpenBabel
       //! \return the residue which contains this atom, or NULL if none exists
       OBResidue *GetResidue(bool perception);
       //! \return the molecule which contains this atom, or NULL if none exists
-      OBMol     *GetParent()        {return((OBMol*)_parent);}
+      OBMol     *GetParent() const        {return((OBMol*)_parent);}
       //! Create a vector for a new bond from this atom, with length given by the supplied parameter
       //! \return success or failure
       bool       GetNewBondVector(vector3 &v,double length);
@@ -486,7 +486,7 @@ namespace OpenBabel
       //! \return Is this atom chiral?
       bool IsChiral();
       //! \return Is the atom part of a periodic unit cell?
-      bool IsPeriodic();
+      bool IsPeriodic() const;
       //! \return Is this atom an axial atom in a ring
       bool IsAxial();
       //! \deprecated
diff --git include/openbabel/bond.h include/openbabel/bond.h
index 838998df..9a168ada 100644
--- include/openbabel/bond.h
+++ include/openbabel/bond.h
@@ -222,14 +222,12 @@ namespace OpenBabel
           return((ptr != _bgn)? _bgn : _end);
         }
       //! \return The enclosing OBMol for this bond, or NULL if none is defined.
-      OBMol  *GetParent()                 {return(_parent);}
+      OBMol  *GetParent() const                 {return(_parent);}
       //! \return The expected "equilibrium" length based on the covalent radii and bond order
       /** Length is given in Angstroms **/
       double  GetEquibLength() const;
       //! \return The current length of this bond in Angstroms
-      //! \todo What is the effect of removing the "const" declaration?
-      //! \todo It appears the problem is that GetLength is const, but OBAtom::GetDistance is not because GetVector is not.
-      double  GetLength();
+      double  GetLength() const;
       //! \return The index to the neighboring atom of @p ptr (i.e., the end if @p ptr is the start)
       /** \warning If @p ptr is not part of the bond, the beginning atom
           index will always be returned **/
@@ -261,7 +259,7 @@ namespace OpenBabel
       bool IsRotor();
       /** \return Is the bond an amide link (i.e., between a carbonyl C and a N)?
            No distinction is made between primary, secondary, and tertiary amides. **/
-      bool IsPeriodic();
+      bool IsPeriodic() const;
       //! \return Is the bond within a periodic unit cell?
       bool IsAmide();
       /** \return Is the bond a primary amide (i.e., between carbonyl C and a NH2)?
diff --git include/openbabel/mol.h include/openbabel/mol.h
index 94c603a0..6d2bbf1c 100644
--- include/openbabel/mol.h
+++ include/openbabel/mol.h
@@ -135,7 +135,7 @@ enum HydrogenType { AllHydrogen, PolarHydrogen, NonPolarHydrogen };
     std::vector<OBInternalCoord*> _internals;   //!< Internal Coordinates (if applicable)
     unsigned short int            _mod;	        //!< Number of nested calls to BeginModify()
 
-    bool  HasFlag(int flag)    { return((_flags & flag) ? true : false); }
+    bool  HasFlag(int flag) const    { return((_flags & flag) ? true : false); }
     void  SetFlag(int flag)    { _flags |= flag; }
 
     //! \name Internal Kekulization routines -- see kekulize.cpp and NewPerceiveKekuleBonds()
@@ -632,7 +632,7 @@ enum HydrogenType { AllHydrogen, PolarHydrogen, NonPolarHydrogen };
     //! Is this molecule chiral?
     bool IsChiral();
     //! Is this molecule periodic? Should periodic boundary conditions be applied?
-    bool IsPeriodic() { return(HasFlag(OB_PERIODIC_MOL)); }
+    bool IsPeriodic() const { return(HasFlag(OB_PERIODIC_MOL)); }
     //! Are there any atoms in this molecule?
     bool Empty()                       { return(_natoms == 0);          }
     //@}
diff --git src/atom.cpp src/atom.cpp
index f7e14a9a..b7a014ee 100644
--- src/atom.cpp
+++ src/atom.cpp
@@ -886,7 +886,7 @@ namespace OpenBabel
     return stereoFacade.HasTetrahedralStereo(_id);
   }
 
-  bool OBAtom::IsPeriodic()
+  bool OBAtom::IsPeriodic() const
   {
     OBMol *mol = (OBMol*) GetParent();
     return mol->IsPeriodic();
diff --git src/bond.cpp src/bond.cpp
index 0d537e9f..f116e272 100644
--- src/bond.cpp
+++ src/bond.cpp
@@ -203,7 +203,7 @@ namespace OpenBabel
     return (_bgn->GetHvyValence() > 1 && _end->GetHvyValence() > 1);
   }
 
-  bool OBBond::IsPeriodic()
+  bool OBBond::IsPeriodic() const
   {
     return ((OBMol*)GetParent())->IsPeriodic();
   }
@@ -746,10 +746,10 @@ namespace OpenBabel
     return(length);
   }
 
-  double OBBond::GetLength()
+  double OBBond::GetLength() const
   {
     double	d2;
-    OBAtom *begin, *end;
+    const OBAtom *begin, *end;
     begin = GetBeginAtom();
     end = GetEndAtom();
 
@@ -762,7 +762,8 @@ namespace OpenBabel
       }
     else
       {
-        return(begin->GetDistance(end));
+        OBUnitCell *box = (OBUnitCell*)GetParent()->GetData(OBGenericDataType::UnitCell);
+        return (box->MinimumImageCartesian(begin->GetVector() - end->GetVector())).length();
       }
   }