diff --git a/src/test.c b/src/test.c
index 3207282..c65e088 100644
--- a/src/test.c
+++ b/src/test.c
@@ -59,6 +59,18 @@ int test_h5write() {
   rc = trexio_write_nucleus_num(file,25);
   assert (rc != TREXIO_SUCCESS);
 
+  // close current session
+  rc = trexio_close(file);
+  assert (rc == TREXIO_SUCCESS);
+
+  file = trexio_open(file_name, 'a', TREXIO_HDF5);
+  assert (file != NULL);
+  
+  // works: try to read the nucleus_num from existing file
+  rc = trexio_read_nucleus_num(file,&num);
+  assert (rc == TREXIO_SUCCESS);
+  assert (num == 12);
+
   // works: try to rewrite the nucleus_coord
   coord[0] = 666.666;
   rc = trexio_write_nucleus_coord(file,coord);
@@ -79,6 +91,7 @@ int test_h5read() {
   int64_t num;
   double* coord;
 
+  // test reading existing file [created by test_h5write()], should work
   file = trexio_open(file_name, 'r', TREXIO_HDF5);
   assert (file != NULL);
 
@@ -100,6 +113,23 @@ int test_h5read() {
   rc = trexio_close(file);
   assert (rc == TREXIO_SUCCESS);
 
+  // test reading non-existing file, should fail and return NULL
+  const char* file_name2 = "test_nonexisting.h5";
+  trexio_t* file2 = NULL;
+
+  file2 = trexio_open(file_name2, 'r', TREXIO_HDF5);
+  assert (file2 == NULL);
+
+  // test appending non-existing file, should create it
+  const char* file_name3 = "test_append.h5";
+  trexio_t* file3 = NULL;
+
+  file3 = trexio_open(file_name3, 'a', TREXIO_HDF5);
+  assert (file3 != NULL);
+
+  rc = trexio_close(file3);
+  assert (rc == TREXIO_SUCCESS);
+
   free(coord);
 
   return 0;
diff --git a/src/trexio.org b/src/trexio.org
index 6363a85..050df1c 100644
--- a/src/trexio.org
+++ b/src/trexio.org
@@ -164,7 +164,7 @@ trexio_t* trexio_open(const char* file_name, const char mode, const back_end_t b
   if (back_end <  0) return NULL;
   if (back_end >= TREXIO_INVALID_BACK_END) return NULL;
   
-  if (mode != 'r' && mode != 'w') return NULL;
+  if (mode != 'r' && mode != 'w' && mode != 'a') return NULL;
 
   trexio_t* result = NULL;
   
diff --git a/src/trexio_hdf5.org b/src/trexio_hdf5.org
index 06e8df2..97d2b91 100644
--- a/src/trexio_hdf5.org
+++ b/src/trexio_hdf5.org
@@ -159,26 +159,59 @@ trexio_exit_code trexio_hdf5_init(trexio_t* file) {
   trexio_hdf5_t* f = (trexio_hdf5_t*) file;
 
   /* If file doesn't exist, create it */
-  int f_ishere = 0;
+  int f_exists = 0;
   struct stat st;
   
-  if (stat(file->file_name, &st) == 0) {
-    printf("%s \n","HDF5 file already exists");
-   // RDWR OR RDONLY ???
-    f->file_id = H5Fopen(file->file_name, H5F_ACC_RDWR, H5P_DEFAULT);
-    f_ishere = 1;
+  if (stat(file->file_name, &st) == 0) f_exists = 1;
+  
+  if (f_exists == 1) {
+
+    switch (file->mode) {
+    case 'r': 
+      // reading the existing file -> open as RDONLY
+      f->file_id = H5Fopen(file->file_name, H5F_ACC_RDONLY, H5P_DEFAULT);
+      break;
+    case 'a': 
+      // appending the existing file -> open as RDWR
+      f->file_id = H5Fopen(file->file_name, H5F_ACC_RDWR, H5P_DEFAULT);
+      break;
+    case 'w': 
+      // writing the existing file -> overwrite it (_TRUNC) [_EXCL | H5F_ACC_DEBUG as an alternative]
+      f->file_id = H5Fcreate(file->file_name, H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
+      break;
+    }  
+
   } else {
-    f->file_id = H5Fcreate(file->file_name, H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
-    f_ishere = 0;
+
+    switch (file->mode) {
+    case 'r': 
+      // reading non-existing file -> error
+      return TREXIO_FAILURE;
+    case 'a': 
+    case 'w': 
+      // appending or writing non-existing file -> create it
+      f->file_id = H5Fcreate(file->file_name, H5F_ACC_EXCL, H5P_DEFAULT, H5P_DEFAULT);
+      break;
+    }  
+
   }  
 
-  /* Create groups in the hdf5 file */
-  if (f_ishere == 0){
-    f->nucleus_group = H5Gcreate(f->file_id, NUCLEUS_GROUP_NAME, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
-    //f->electron_group = H5Gcreate(f->file_id, ELECTRON_GROUP_NAME, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
-  } else { 
-    f->nucleus_group = H5Gopen(f->file_id, NUCLEUS_GROUP_NAME, H5P_DEFAULT);
-    //f->electron_group = H5Gopen(f->file_id, ELECTRON_GROUP_NAME, H5P_DEFAULT);   
+  /* Create or open groups in the hdf5 file assuming that they exist if file exists */    
+  switch (file->mode) {
+    case 'r':
+    case 'a': 
+      if (f_exists == 1) {
+        f->nucleus_group = H5Gopen(f->file_id, NUCLEUS_GROUP_NAME, H5P_DEFAULT);
+      //f->electron_group = H5Gopen(f->file_id, ELECTRON_GROUP_NAME, H5P_DEFAULT);   
+      } else {
+        f->nucleus_group = H5Gcreate(f->file_id, NUCLEUS_GROUP_NAME, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+      //f->electron_group = H5Gcreate(f->file_id, ELECTRON_GROUP_NAME, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+      }
+      break;
+    case 'w':
+      f->nucleus_group = H5Gcreate(f->file_id, NUCLEUS_GROUP_NAME, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+      //f->electron_group = H5Gcreate(f->file_id, ELECTRON_GROUP_NAME, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+      break;
   }
   assert (f->nucleus_group > 0L);
   //assert (f->electron_group > 0L);
@@ -578,7 +611,6 @@ trexio_exit_code trexio_hdf5_write_nucleus_coord(const trexio_t* file, const dou
 }
 
     #+end_src
-** TODO Read/Write the charge attribute
 
 * File suffixes                                                     :noxport: