diff --git a/src/Makefile b/src/Makefile
index f72ee11..53d4a3f 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -49,13 +49,13 @@ fortran: libtrexio.so trexio_f.f90
 	$(FC) $(FFLAGS) -c trexio_f.f90 -o trexio_f.o
 
 test: libtrexio.so test.c test.f90 trexio_f.o
-	rm -f -r trexio_test/
+	rm -f -r test_text_fort/
 	$(CC) $(CFLAGS) $(INCLUDE) -Wl,-rpath,$(PWD) -L. test.c -ltrexio $(LIBS) -o test
 	$(CC) $(CFLAGS) $(INCLUDE) -Wl,-rpath,$(PWD) -L. test.f90 -ltrexio $(LIBS) -o test_f
 
 clean:
 	rm -f *.o libtrexio.so test test_*.h5 test_f
-	rm -r trexio_test/
+	rm -r test_text_fort/
 
 %.o: %.c $(HEADER_FILES)
 	$(CC) $(CFLAGS) $(INCLUDE) -c $*.c -o $*.o
diff --git a/src/test.f90 b/src/test.f90
index 15dd5a9..1072eb1 100644
--- a/src/test.f90
+++ b/src/test.f90
@@ -1,24 +1,31 @@
 program test_trexio
 
-  use trexio
+  call test_write()
+  call test_read()
 
+end program test_trexio
+
+subroutine test_write()
+   
+  use trexio
+  use, intrinsic :: iso_c_binding
   implicit none
 
-  type(c_ptr) :: file1
+  type(c_ptr) :: trex_file
 
   integer :: rc 
   integer (c_int64_t) :: num
 
-  type(c_ptr) :: num_test 
-  integer, pointer :: num_read
-  integer, target :: num666
-
   real, dimension(12), target :: charge
-  real, dimension(36) :: coord
+  real, dimension(36), target :: coord
 
   type(c_ptr) :: charge_ptr
   real(c_double), pointer :: charge_cp(:)
 
+  type(c_ptr) :: coord_ptr
+  real(c_double), pointer :: coord_cp(:)
+
+
   num = 12
 
   charge = (/6., 6., 6., 6., 6., 6., 1., 1., 1., 1., 1., 1. /)
@@ -40,28 +47,101 @@ program test_trexio
              2.14171677 ,  1.23652075 ,  0.00000000 , &
              0.00000000 ,  2.47304151 ,  0.00000000 /)
 
+  allocate(coord_cp(size(coord)))
+  coord_cp = coord
+  coord_ptr = c_loc(coord_cp)
+
+  rc = 0
+
+  trex_file = trexio_open('test_text_fort' // c_null_char, 'w', 1)
+!  trex_file = trexio_open('test_hdf5_fort.h5' // c_null_char, 'w', 0)
+
+  rc = trexio_write_nucleus_num(trex_file, num)
+  if (rc == 0) write(*,*) 'SUCCESS WRITE NUM'
+
+  rc = trexio_write_nucleus_charge(trex_file, charge_ptr)
+  if (rc == 0) write(*,*) 'SUCCESS WRITE CHARGE'
+
+  rc = trexio_write_nucleus_coord(trex_file, coord_ptr)
+  if (rc == 0) write(*,*) 'SUCCESS WRITE COORD'
+
+  rc = trexio_close(trex_file)
+  if (rc == 0) write(*,*) 'SUCCESS CLOSE' 
+
+! ---------------------------------- !
+! to modify fiels of existing file:
+! text backend -> open with 'w'
+! hdf5 backend -> open with 'a'
+! ---------------------------------- !
+
+  trex_file = trexio_open('test_text_fort' // c_null_char, 'w', 1);
+!  trex_file = trexio_open('test_hdf5_fort.h5' // c_null_char, 'a', 0)
+  
+  coord(1) = 666.666
+  coord_cp = coord
+  coord_ptr = c_loc(coord_cp)
+
+  rc = trexio_write_nucleus_coord(trex_file,coord_ptr)
+  if (rc == 0) write(*,*) 'SUCCESS MODIFY COORD'
+
+  rc = trexio_close(trex_file)
+  if (rc == 0) write(*,*) 'SUCCESS CLOSE 2' 
+
+
+  deallocate(charge_cp)
+  deallocate(coord_cp)
+
+end subroutine test_write
+
+subroutine test_read()
+
+  use trexio
+  use, intrinsic :: iso_c_binding
+  implicit none
+
+  type(c_ptr) :: trex_file
+
+  integer :: rc 
+  integer (c_int64_t) :: num
+
+  type(c_ptr) :: num_test 
+  integer, pointer :: num_read
+  integer, target :: num666
+
+  real, dimension(12) :: charge
+
+  type(c_ptr) :: charge_ptr
+  real(c_double), pointer :: charge_read(:)
+
+  num = 12
+
+  charge = (/0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0. /)
+
+  allocate(charge_read(size(charge)))
+  charge_read = charge
+  charge_ptr = c_loc(charge_read)
+
   rc = 0
 
   num_test = c_loc(num666)
 
-  file1 = trexio_open('trexio_test' // c_null_char, 'w', 1)
-!  file1 = trexio_open('test_fort.h5' // c_null_char, 'w', 0)
-
-  rc = trexio_write_nucleus_num(file1, num)
-  if (rc == 0) write(*,*) 'SUCCESS WRITE NUM'
-
-  rc = trexio_write_nucleus_charge(file1, charge_ptr)
-  if (rc == 0) write(*,*) 'SUCCESS WRITE CHARGE'
-
-  rc = trexio_read_nucleus_num(file1, num_test)
+  trex_file = trexio_open('test_text_fort' // c_null_char, 'r', 1)
+!  trex_file = trexio_open('test_hdf5_fort.h5' // c_null_char, 'r', 0)
 
+  rc = trexio_read_nucleus_num(trex_file, num_test)
   call c_f_pointer(num_test, num_read)
 
-  if (rc == 0 .and. num_read == num) write(*,*) 'SUCCESS READ'
+  if (rc == 0 .and. num_read == num) write(*,*) 'SUCCESS READ NUM'
 
-  rc = trexio_close(file1)
+  rc = trexio_read_nucleus_charge(trex_file, charge_ptr)
+  call c_f_pointer(charge_ptr, charge_read, [num_read])
+   
+  if (rc == 0 .and. (abs (charge_read(11) - 1.0) < 1.0D-8) ) write(*,*) 'SUCCESS READ CHARGE'
+
+  rc = trexio_close(trex_file)
   if (rc == 0) write(*,*) 'SUCCESS CLOSE' 
 
-  deallocate(charge_cp)
+  deallocate(charge_read)
+
+end subroutine test_read
 
-end program test_trexio