diff --git a/src/Makefile b/src/Makefile
new file mode 100644
index 0000000..16bf723
--- /dev/null
+++ b/src/Makefile
@@ -0,0 +1,52 @@
+COMPILER=GNU
+#COMPILER=INTEL
+#COMPILER=LLVM
+
+ifeq ($(COMPILER),GNU)
+CC=gcc -g
+CFLAGS=-fPIC -fexceptions -Wall -Werror -Wpedantic -Wextra
+
+LIBS= #-lgfortran -lm
+endif
+
+ifeq ($(COMPILER),INTEL)
+CC=icc -xHost
+CFLAGS=-fPIC -g -O2
+
+FC=ifort -xHost
+FFLAGS=-fPIC -g -O2
+
+LIBS= #-lm 
+endif
+
+#TODO
+ifeq ($(COMPILER),LLVM)
+CC=clang
+CFLAGS=-fPIC -g -O2
+
+FC=flang
+FFLAGS=fPIC -g -O2
+
+LIBS=-lm
+endif
+
+
+OBJECT_FILES= trio.o trio_text.o
+HEADER_FILES= trio.h trio_text.h trio_s.h
+
+export CC CFLAGS LIBS
+
+.PHONY: clean
+
+libtrio.so: $(OBJECT_FILES) $(HEADER_FILES)
+	$(CC) -shared $(OBJECT_FILES) -o libtrio.so
+
+test: libtrio.so test.c
+	$(CC) $(CFLAGS) -Wl,-rpath,$(PWD) -L. test.c -ltrio $(LIBS) -o test
+
+clean:
+	rm -f *.o libtrio.so
+
+%.o: %.c $(HEADER_FILES)
+	$(CC) $(CFLAGS) -c $*.c -o $*.o
+
diff --git a/src/trio.org b/src/trio.org
new file mode 100644
index 0000000..4c86f65
--- /dev/null
+++ b/src/trio.org
@@ -0,0 +1,684 @@
+#+TITLE: TREX Input/Ouput library (TRIO)
+
+* File prefixes                                                     :noxport:
+
+  #+begin_src c :tangle trio.h
+#ifndef _TRIO_H
+#define _TRIO_H
+
+#include <stdint.h>
+
+  #+end_src
+  
+  #+begin_src c :tangle trio.c
+#include <pthread.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+#include <pthread.h>
+
+#include "trio.h"
+#include "trio_s.h"
+#include "trio_text.h"
+/*
+#include "trio_hdf5.h"
+#include "trio_json.h"
+,*/
+
+  #+end_src
+
+
+  #+begin_src c :tangle trio_s.h
+#ifndef _TRIO_S_H
+#define _TRIO_S_H
+
+#include "trio.h"
+#include <pthread.h>
+#include <assert.h>
+  #+end_src
+
+  #+begin_src c :tangle trio_text.h
+#ifndef _TRIO_TEXT_H
+#define _TRIO_TEXT_H
+
+#include "trio.h"
+#include "trio_s.h"
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <assert.h>
+
+  #+end_src
+  
+  #+begin_src c :tangle trio_text.c
+#include "trio_text.h"
+  #+end_src
+
+* Coding conventions
+
+  - integer types will be defined using types given in ~stdint.h~
+  - pointers are always initialized to ~NULL~
+  - when memory is freed, the pointer is set to ~NULL~
+  - ~assert.h~ should be used extensively
+  - variable names are in lower case
+  - ~#define~ constants are in upper case
+  - structs are suffixed by ~_s~
+  - types are suffixed by ~_t~
+
+* Front end
+
+ All calls to TRIO are thread-safe.
+
+** Error handling
+   #+begin_src c :tangle trio.h
+typedef int32_t trio_exit_code;
+
+#define TRIO_SUCCESS ( (trio_exit_code) 0 )
+#define TRIO_FAILURE ( (trio_exit_code) 1 )
+   #+end_src
+   
+   
+** Back ends
+
+   #+begin_src c :tangle trio.h
+typedef uint32_t back_end_t;
+
+#define TRIO_HDF5             ( (back_end_t) 0 )
+#define TRIO_TEXT             ( (back_end_t) 1 ) 
+#define TRIO_JSON             ( (back_end_t) 2 ) 
+#define TRIO_INVALID_BACK_END ( (back_end_t) 3 )
+   #+end_src
+
+** Read/write behavior
+   
+   Every time a reading function is called, the data is read from the
+   disk. If data needs to be cached, this is left to the user of the
+   library.
+ 
+   Writing to TRIO files is done with transactions (all-or-nothing
+   effect) in a per-group fashion.  File writes are attempted by
+   calling explicitly the flush function, or when the TRIO file is
+   closed. If writing is impossible because the data is not valid, no
+   data is written.
+
+   The order in which the data is written is not necessarily consistent
+   with the order in which the function calls were made.
+
+   The TRIO files are supposed to be opened by only one program at a
+   time: if the same TRIO file is modified simultaneously by multiple
+   concurrent programs, the behavior is not specified.
+ 
+** TRIO file type
+   
+   ~trio_s~ is the the main type for TRIO files, visible to the users
+   of the library. This type is kept opaque, and all modifications to
+   the files will be necessarily done through the use of functions,
+   taking such a type as argument.
+  
+   File creation and opening functions will return /TRIO file handles/,
+   namely pointers to ~trio_s~ types. All functions accessing to the
+   TRIO files will have as a first argument the TRIO file handle.
+
+   #+begin_src c :tangle trio.h
+typedef struct trio_s trio_t;
+   #+end_src
+   
+   #+begin_src c :tangle trio_s.h
+struct trio_s {
+  char*             file_name;
+  pthread_mutex_t   thread_lock;
+  back_end_t        back_end;
+  char              mode;
+  char              padding[7];   /* Ensures the proper alignment of back-ends */
+};
+   #+end_src
+
+** Polymorphism of the file handle
+   
+   Polymorphism of the ~trio_t~ type is handled by ensuring that the
+   corresponding types for all back ends can be safely casted to
+   ~trio_t~. This is done by making the back end structs start with
+   ~struct trio_s~:
+
+   #+begin_src c 
+struct trio_back_end_s {
+  trio_t     parent ;
+  /* add below specific back end data */
+} 
+   #+end_src
+   
+** File creation and opening
+   
+   #+begin_src c :tangle trio.h
+trio_t* trio_create(const char* file_name, back_end_t back_end);
+   #+end_src
+
+   
+   #+begin_src c :tangle trio.c
+trio_t* trio_create(const char* file_name, back_end_t back_end) {
+
+  /* Check that file name is not NULL or empty */
+  assert (file_name != NULL);
+  assert (file_name[0] != '\0');
+  
+  /* Check that back_end is valid */
+  assert (back_end < TRIO_INVALID_BACK_END);
+  
+  trio_t* result = NULL;
+  
+  switch (back_end) {
+
+  case TRIO_TEXT:
+    result = (trio_t*) malloc (sizeof(trio_text_t));
+    break;
+/*
+  case TRIO_HDF5:
+    result = (trio_t*) malloc (sizeof(trio_hdf5_t));
+    break;
+
+  case TRIO_JSON:
+    result = (trio_t*) malloc (sizeof(trio_json_t));
+    break;
+,*/      
+  default:
+    assert (1 == 0);  /* Impossible case */
+  }
+  
+  /* TODO: Error handling */
+  assert (result != NULL);
+
+  result->file_name   = (char*) calloc(strlen(file_name)+1,sizeof(char));
+  strcpy(result->file_name, file_name);
+  result->back_end    = back_end;
+  result->mode        = 'w';  /* Upon creation, mode=write */
+  pthread_mutex_init ( &(result->thread_lock), NULL);
+
+  trio_exit_code rc = TRIO_FAILURE;
+  
+  switch (back_end) {
+
+  case TRIO_TEXT:
+    rc = trio_text_init(result);
+    break;
+/*
+  case TRIO_HDF5:
+    rc = trio_hdf5_init(result);
+    break;
+
+  case TRIO_JSON:
+    rc = trio_json_init(result);
+    break;
+*/      
+  default:
+    assert (1 == 0);  /* Impossible case */
+  }
+  assert (rc == TRIO_SUCCESS);
+  
+  return result;
+}
+   #+end_src
+   
+** Reading/writing data
+
+   #+begin_src c :tangle trio.h
+trio_exit_code trio_read_nucleus_num(trio_t* file, int64_t* num);
+   #+end_src
+   
+   #+begin_src c :tangle trio.c
+trio_exit_code trio_read_nucleus_num(trio_t* file, int64_t* num) {
+  if (file == NULL) return TRIO_FAILURE;
+
+  switch (file->back_end) {
+
+  case TRIO_TEXT:
+    return trio_text_read_nucleus_num(file, num);
+    break;
+/*
+  case TRIO_HDF5:
+    return trio_hdf5_read_nucleus_num(file, num);
+    break;
+
+  case TRIO_JSON:
+    return trio_json_read_nucleus_num(file, num);
+    break;
+*/      
+  default:
+    return TRIO_FAILURE;  /* Impossible case */
+  }
+}
+   #+end_src
+
+
+   #+begin_src c :tangle trio.h
+trio_exit_code trio_read_nucleus_coord(trio_t* file, double* coord);
+   #+end_src
+   
+   #+begin_src c :tangle trio.c
+trio_exit_code trio_read_nucleus_coord(trio_t* file, double* coord) {
+  if (file == NULL) return TRIO_FAILURE;
+
+  switch (file->back_end) {
+
+  case TRIO_TEXT:
+    return trio_text_read_nucleus_coord(file, coord);
+    break;
+/*
+  case TRIO_HDF5:
+    return trio_hdf5_read_nucleus_coord(file, coord);
+    break;
+
+  case TRIO_JSON:
+    return trio_json_read_nucleus_coord(file, coord);
+    break;
+*/
+  default:
+    return TRIO_FAILURE;  /* Impossible case */
+  }
+}
+   #+end_src
+
+
+   #+begin_src c :tangle trio.h
+trio_exit_code trio_read_nucleus_charge(trio_t* file, double* charge);
+   #+end_src
+   
+   #+begin_src c :tangle trio.c
+trio_exit_code trio_read_nucleus_charge(trio_t* file, double* charge) {
+  if (file == NULL) return TRIO_FAILURE;
+
+  switch (file->back_end) {
+
+  case TRIO_TEXT:
+    return trio_text_read_nucleus_charge(file, charge);
+    break;
+/*
+  case TRIO_HDF5:
+    return trio_hdf5_read_nucleus_charge(file, charge);
+    break;
+
+  case TRIO_JSON:
+    return trio_json_read_nucleus_charge(file, charge);
+    break;
+*/
+  default:
+    return TRIO_FAILURE;  /* Impossible case */
+  }
+}
+   #+end_src
+   
+* Back ends
+  
+   TRIO has multiple possible back ends:
+
+   - HDF5: The most efficient back-end, by default
+   - Text files: not to be used for production, but useful for debugging
+   - JSON: for portability
+     
+** TEXT Back end
+
+  #+begin_src c :tangle trio_text.h
+typedef struct nucleus_s {
+  double*  coord;
+  double*  charge;
+  int64_t  num;
+} nucleus_t;
+
+typedef struct electron_s {
+  int64_t  alpha_num;
+  int64_t  beta_num;
+} electron_t;
+
+typedef struct trio_text_s {
+  trio_t     parent ;
+  char*      nucleus_file_name;
+  char*      electron_file_name;
+} trio_text_t;
+
+  #+end_src
+
+
+  #+begin_src c :tangle trio_text.h
+trio_exit_code trio_text_init(trio_t* file);
+  #+end_src
+  
+  #+begin_src c :tangle trio_text.c
+trio_exit_code trio_text_init(trio_t* file) {
+
+  trio_text_t* f = (trio_text_t*) file;
+
+  const char* nucleus_file_name = "/nucleus.txt";
+  f->nucleus_file_name = (char*)
+    calloc( strlen(file->file_name) + strlen(nucleus_file_name) + 1,
+            sizeof(char));
+  assert (f->nucleus_file_name != NULL);
+  strcat (f->nucleus_file_name, nucleus_file_name);
+
+
+  const char* electron_file_name = "/electron.txt";
+  f->nucleus_file_name = (char*)
+    calloc( strlen(file->file_name) + strlen(electron_file_name) + 1,
+            sizeof(char));
+  assert (f->nucleus_file_name != NULL);
+  strcat (f->nucleus_file_name, electron_file_name);
+
+  return TRIO_SUCCESS;
+}
+
+  #+end_src
+
+    
+  #+begin_src c :tangle trio_text.h
+trio_exit_code trio_text_finalize(trio_t* file);
+  #+end_src
+
+  #+begin_src c :tangle trio_text.c
+trio_exit_code trio_text_finalize(trio_t* file) {
+
+  trio_text_t* f = (trio_text_t*) file;
+
+  free (f->nucleus_file_name);
+  f->nucleus_file_name = NULL;
+
+  free (f->electron_file_name);
+  f->electron_file_name = NULL;
+
+  return TRIO_SUCCESS;
+}
+  #+end_src
+
+  
+  
+*** Read/write the nucleus struct
+
+   #+begin_src c :tangle trio_text.c
+nucleus_t* trio_text_read_nucleus(const trio_text_t* file) {
+
+  /* Allocate the data structure */
+  nucleus_t* nucleus = (nucleus_t*) malloc(sizeof(nucleus_t));
+  assert (nucleus != NULL);
+
+  nucleus->num    = 0;
+  nucleus->coord  = NULL;
+  nucleus->charge = NULL;
+
+  /* Try to open the file. If the file does not exist, return */
+  FILE* f = fopen(file->nucleus_file_name,"r");
+  if (f == NULL) {
+    return nucleus;
+  }
+
+  /* Find size of file to allocate the max size of the string buffer */
+  fseek(f, 0L, SEEK_END);
+  size_t sz = ftell(f);
+  fseek(f, 0L, SEEK_SET);
+  char* buffer = (char*) malloc(sz*sizeof(char));
+
+  /* Read the dimensioning variables */
+  fscanf(f, "%s", buffer);
+  assert (strcmp(buffer, "num") == 0);
+
+  fscanf(f, "%ld", &(nucleus->num));
+  assert (nucleus->num > 0);
+
+  /* Allocate arrays */
+  nucleus->charge = (double*) calloc(nucleus->num, sizeof(double));
+  assert (nucleus->charge != NULL);
+  
+  nucleus->coord = (double*) calloc(3 * nucleus->num, sizeof(double));
+  assert (nucleus->coord != NULL);
+
+  /* Read arrays */
+  fscanf(f, "%s", buffer);
+  assert (strcmp(buffer, "charge") == 0);
+
+  for (int i=0 ; i<nucleus->num ; i++) {
+    fscanf(f, "%lf", &(nucleus->charge[i]));
+  }
+  
+  fscanf(f, "%s", buffer);
+  assert (strcmp(buffer, "coord") == 0);
+
+  for (int i=0 ; i<3*nucleus->num ; i++) {
+      fscanf(f, "%lf", &(nucleus->coord[i]));
+  }
+  free(buffer);
+  fclose(f);
+  return nucleus;
+}
+
+
+trio_exit_code trio_text_write_nucleus(const trio_text_t* file, nucleus_t* nucleus) {
+  assert (nucleus != NULL);
+
+  FILE* f = fopen(file->nucleus_file_name,"w");
+  assert (f != NULL);
+
+  /* Write the dimensioning variables */
+  fprintf(f, "num %ld\n", nucleus->num);
+
+  /* Write arrays */
+  fprintf(f, "charge\n");
+  for (int i=0 ; i<nucleus->num ; i++) {
+    fprintf(f, "%lf\n", nucleus->charge[i]);
+  }
+
+  fprintf(f, "coord\n");
+  for (int i=0 ; i<3*nucleus->num ; i++) {
+      fprintf(f, "%lf\n", nucleus->coord[i]);
+  }
+
+  fclose(f);
+  return TRIO_SUCCESS;
+}
+  #+end_src
+
+*** Free memory
+
+    Memory is allocated when reading. The followig function frees memory.
+    
+   #+begin_src c :tangle trio_text.c
+trio_exit_code trio_text_free_nucleus(nucleus_t* nucleus) {
+  
+  if (nucleus == NULL) {
+    return TRIO_FAILURE;
+  }
+  
+  if (nucleus->coord != NULL) {
+    free (nucleus->coord);
+  }
+  nucleus->coord = NULL;
+  
+  if (nucleus->charge != NULL) {
+    free (nucleus->charge);
+  }
+  nucleus->charge = NULL;
+  
+  free (nucleus);
+  return TRIO_SUCCESS;
+}
+   #+end_src
+
+*** Read/Write the num attribute
+
+   #+begin_src c :tangle trio_text.h
+trio_exit_code trio_text_read_nucleus_num(const trio_t* file, int64_t* num);
+trio_exit_code trio_text_write_nucleus_num(const trio_t* file, const int64_t num);
+  #+end_src
+
+   #+begin_src c :tangle trio_text.c
+trio_exit_code trio_text_read_nucleus_num(const trio_t* file, int64_t* num) {
+
+  assert (file != NULL);
+  assert (num  != NULL);
+
+  nucleus_t* nucleus = trio_text_read_nucleus((trio_text_t*) file);
+  
+  if (nucleus == NULL) {
+    return TRIO_FAILURE;
+  }
+
+  /**/ *num = nucleus->num;
+
+  trio_text_free_nucleus(nucleus);
+  return TRIO_SUCCESS;
+}
+
+ 
+trio_exit_code trio_text_write_nucleus_num(const trio_t* file, const int64_t num) {
+
+  assert (num > 0L);
+  assert (file != NULL);
+  
+  nucleus_t* nucleus = trio_text_read_nucleus((trio_text_t*) file);
+
+  assert (nucleus != NULL);
+  
+  if (nucleus->num != num) {
+
+    nucleus->num = num;
+
+    if (nucleus->charge != NULL) free(nucleus->charge);
+    nucleus->charge = NULL;
+
+    nucleus->charge = (double*) calloc(num, sizeof(double));
+    assert (nucleus->charge != NULL);
+    
+    if (nucleus->coord  != NULL) free(nucleus->coord );
+    nucleus->coord = NULL;
+
+    nucleus->coord = (double*) calloc(3*num, sizeof(double));
+    assert (nucleus->coord != NULL);
+
+  } else {
+    nucleus->num = num;
+  }
+  
+  trio_exit_code rc = trio_text_write_nucleus((trio_text_t*) file, nucleus);
+  assert (rc == TRIO_SUCCESS);
+
+  trio_text_free_nucleus(nucleus);
+  
+  return TRIO_SUCCESS;
+}
+    #+end_src
+
+*** Read/Write the coord attribute
+
+    The ~coord~ array is assumed allocated with the appropriate size.
+    
+   #+begin_src c :tangle trio_text.h
+trio_exit_code trio_text_read_nucleus_coord(const trio_t* file, double* coord);
+trio_exit_code trio_text_write_nucleus_coord(const trio_t* file, const double* coord);
+  #+end_src
+
+   #+begin_src c :tangle trio_text.c
+trio_exit_code trio_text_read_nucleus_coord(const trio_t* file, double* coord) {
+
+  assert (file != NULL);
+  assert (file != NULL);
+  nucleus_t* nucleus = trio_text_read_nucleus((trio_text_t*) file);
+  
+  if (nucleus == NULL) {
+    return TRIO_FAILURE;
+  }
+
+  assert (coord != NULL);
+  
+  for (int i=0 ; i<3*nucleus->num ; i++) {
+    coord[i] = nucleus->coord[i];
+  }
+
+  trio_text_free_nucleus(nucleus);
+  return TRIO_SUCCESS;
+}
+
+ 
+trio_exit_code trio_text_write_nucleus_coord(const trio_t* file, const double* coord) {
+
+  assert (coord != NULL);
+  assert (file != NULL);
+  
+  nucleus_t* nucleus = trio_text_read_nucleus((trio_text_t*) file);
+  assert (nucleus != NULL);
+  
+  for (int i=0 ; i<3*nucleus->num ; i++) {
+    nucleus->coord[i] = coord[i];
+  }
+  
+  trio_exit_code rc = trio_text_write_nucleus((trio_text_t*) file, nucleus);
+  assert (rc == TRIO_SUCCESS);
+
+  trio_text_free_nucleus(nucleus);
+  
+  return TRIO_SUCCESS;
+}
+    #+end_src
+*** Read/Write the charge attribute
+
+    The ~charge~ array is assumed allocated with the appropriate size.
+    
+   #+begin_src c :tangle trio_text.h
+trio_exit_code trio_text_read_nucleus_charge(const trio_t* file, double* coord);
+trio_exit_code trio_text_write_nucleus_charge(const trio_t* file, const double* coord);
+  #+end_src
+
+   #+begin_src c :tangle trio_text.c
+trio_exit_code trio_text_read_nucleus_charge(const trio_t* file, double* charge) {
+
+  assert (file != NULL);
+  assert (file != NULL);
+  nucleus_t* nucleus = trio_text_read_nucleus((trio_text_t*)file);
+  
+  if (nucleus == NULL) {
+    return TRIO_FAILURE;
+  }
+
+  assert (charge != NULL);
+  
+  for (int i=0 ; i<nucleus->num ; i++) {
+    charge[i] = nucleus->charge[i];
+  }
+
+  trio_text_free_nucleus(nucleus);
+  return TRIO_SUCCESS;
+}
+
+ 
+trio_exit_code trio_text_write_nucleus_charge(const trio_t* file, const double* charge) {
+
+  assert (charge != NULL);
+  assert (file != NULL);
+  
+  nucleus_t* nucleus = trio_text_read_nucleus((trio_text_t*)file);
+  assert (nucleus != NULL);
+  
+  for (int i=0 ; i<nucleus->num ; i++) {
+    nucleus->charge[i] = charge[i];
+  }
+  
+  trio_exit_code rc = trio_text_write_nucleus((trio_text_t*) file, nucleus);
+  assert (rc == TRIO_SUCCESS);
+
+  trio_text_free_nucleus(nucleus);
+  
+  return TRIO_SUCCESS;
+}
+    #+end_src
+** HDF5 Back end
+* File suffixes                                                     :noxport:
+
+  #+begin_src c :tangle trio.h
+#endif
+  #+end_src
+
+  #+begin_src c :tangle trio_s.h
+#endif
+  #+end_src
+
+  #+begin_src c :tangle trio_text.h
+#endif
+  #+end_src
+* TODO Things to be done                                           :noexport:
+  - [ ] Thread safety
+  - [ ] Error handling with errno