diff --git a/structure/README.md b/structure/README.md
index 2f72ff6..e50a1f6 100644
--- a/structure/README.md
+++ b/structure/README.md
@@ -27,6 +27,7 @@ A tutorial for the protein structure modules of [BioJava](http://www.biojava.org
 
 This tutorial is split into several chapters.
 
+
 Chapter 1 - Quick [Installation](installation.md)
 
 Chapter 2 - [First Steps](firststeps.md)
@@ -47,13 +48,20 @@ Chapter 9 - [Biological Assemblies](bioassembly.md)
 
 Chapter 10 - [External Databases](externaldb.md) like SCOP & CATH
 
-Chapter 11 - Protein Symmetry
+Chapter 11 - [Accessible Surface Areas](asa.md)
+
+Chapter 12 - [Contacts within a chain and between chains](contact-map.md)
+
+Chapter 13 - Finding all interfaces in crystal: [crystal contacts](crystal-contacts.md)
+
+Chapter 14 - Protein Symmetry
+
+Chapter 15 - Bonds
 
-Chapter 12 - Bonds
+Chapter 16 - [Special Cases](special.md)
 
-Chapter 13 - [Special Cases](special.md)
+Chapter 17 - [Lists](lists.md) of PDB IDs and PDB [status information](lists.md).
 
-Chapter 14 - [Lists](lists.md) of PDB IDs and PDB [status information](lists.md).
 
 ### Author: 
 
diff --git a/structure/asa.md b/structure/asa.md
new file mode 100644
index 0000000..792b044
--- /dev/null
+++ b/structure/asa.md
@@ -0,0 +1,36 @@
+# Calculating Accessible Surface Areas
+
+BioJava can also do calculation of Accessible Surface Areas (ASA) through an implementation of the rolling ball algorithm of Shrake and Rupley [Shrake 1973].
+
+This code will do the ASA calculation and output the values per residue and the total:
+```java
+		AtomCache cache = new AtomCache();
+		cache.setUseMmCif(true);
+		
+		StructureIO.setAtomCache(cache); 
+		
+		Structure structure = StructureIO.getStructure("1smt");
+		
+		AsaCalculator asaCalc = new AsaCalculator(structure, 
+				AsaCalculator.DEFAULT_PROBE_SIZE, 
+				1000, 1, false);
+		
+		GroupAsa[] groupAsas = asaCalc.getGroupAsas();
+		
+		double tot = 0;
+		
+		for (GroupAsa groupAsa: groupAsas) {
+			System.out.printf("%1s\t%5s\t%3s\t%6.2f\n", 
+					groupAsa.getGroup().getChainId(),
+					groupAsa.getGroup().getResidueNumber(),
+					groupAsa.getGroup().getPDBName(), 
+					groupAsa.getAsaU());
+			tot+=groupAsa.getAsaU();
+		}
+		
+		System.out.printf("Total area: %9.2f\n",tot);
+		
+```
+See [DemoAsa](https://github.com/biojava/biojava/blob/master/biojava3-structure/src/main/java/demo/DemoAsa.java) for a fully working demo.
+
+[Shrake 1973]: http://www.sciencedirect.com/science/article/pii/0022283673900119
diff --git a/structure/crystal-contacts.md b/structure/crystal-contacts.md
index 97ba9bb..9dfa1af 100644
--- a/structure/crystal-contacts.md
+++ b/structure/crystal-contacts.md
@@ -44,4 +44,17 @@ The algorithm to find all unique interfaces in the crystal works roughly like th
 
 See [DemoCrystalInterfaces](https://github.com/biojava/biojava/blob/master/biojava3-structure/src/main/java/demo/DemoCrystalInterfaces.java) for a fully working demo of the example above.
 
+## Clustering the interfaces
+One can also cluster the interfaces based on their similarity. The similarity is measured through contact overlap: number of common contacts over average number of contact in both chains. The clustering can be done as following:
+
+```java
+		List<StructureInterfaceCluster> clusters = interfaces.getClusters();
+		for (StructureInterfaceCluster cluster:clusters) {
+			System.out.print("Cluster "+cluster.getId()+" members: ");
+			for (StructureInterface member:cluster.getMembers()) {
+				System.out.print(member.getId()+" ");
+			}
+			System.out.println();
+		}
+```