diff --git a/src/pdb/protein.rs b/src/pdb/protein.rs
index 7199a88b008144db154a2dff09969dc716dd7b0f..be05aa80a263f0ba1ae0fc6ab87d1c657c39f542 100644
--- a/src/pdb/protein.rs
+++ b/src/pdb/protein.rs
@@ -175,4 +175,30 @@ impl<'a> Protein {
         let atom = Atom::new(atom_name, atom_number, coord);
         residue.add_atom(atom);
     }
+
+    /// function that return a vector for atom index
+    /// Can be used in other program like rrmsd_map to select specific atom
+    /// 
+    /// # Examples
+    /// ```
+    /// use pdbparser;
+    /// 
+    /// let my_prot = pdbparser::open_pdb_file("src/tests/f2_adn.pdb");
+    /// let atom_index = my_prot.get_atom_index();
+    /// 
+    /// assert_eq!(atom_index.next(), 1);
+    /// assert_eq!(atom_index.next(), 2);
+    /// 
+    /// ```
+    pub fn get_atom_index(&self) -> Vec<u64> {
+        let mut lst: Vec<u64> = Vec::new();
+        for chain in &self.lst_chain {
+            for res in &chain.lst_res {
+                for atom in &res.lst_atom {
+                    lst.push(atom.number);
+                }
+            }
+        }
+        lst
+    }
 }