Clarify send/receiving data for MDI

driver_tutorial.md

@@ -315,10 +315,19 @@ When using MDI, we retrieve data from the engine by sending a command to the eng
 The first command we use is the `mdi.MDI_Send_Command` function. 
 The argument to this function is the command to send and the engine we would like to send the command to.
 When using `MDI_Send_Command`, we use `<` to tell the engine to send data to the driver, and `>` to tell the driver to send data to the engine.
-
 For example, to get the number of atoms in the system, we first call `mdi.MDI_Send_Command("<NATOMS", lammps)` to tell LAMMPS to send the number of atoms.
-Then, we call `mdi.MDI_Recv` to receive the data.
+
+After we have sent the command to the engine, we have to retrieve the data.
+This is done with the `mdi.MDI_Recv` function.
 When using `mdi.MDI_Recv`, we have to tell MDI how many values we expect to get back and the data type for the value or values we expect to receive.
+They syntax is
+
+```python
+mdi.MDI_Recv(EXPECTED_NUMBER_OF_VALUES, DATA_TYPE, ENGINE_COMMUNICATOR)
+```
+
+The expected data types are defined in the MDI library.
+For example, `mdi.MDI_INT` is the data type for an integer, and `mdi.MDI_DOUBLE` is the data type for a double (decimal) value.
 
 Add the following code to the `# Perform the simulation` section to get the number of atoms in the system:
 
@@ -336,14 +345,16 @@ print(f"The number of atoms in the system is {natoms}.")
 ```
 ````
 
-These set of commands first tells LAMMPS to send the number of atoms in the system (`mdi.MDI_Send_Command("<NATOMS", lammps)`). After that, we receive the number of atoms from LAMMPS. Using `mdi.MDI_Recv`, we tell lammps we are expecting `1` value for `natoms` and that its data type is an integer.
+These set of commands first tells LAMMPS to send the number of atoms in the system (`mdi.MDI_Send_Command("<NATOMS", lammps)`). 
+After that, we receive the number of atoms from LAMMPS. Using `mdi.MDI_Recv`, we tell lammps we are expecting `1` value for `natoms` and that its data type is an integer.
 
 ````{admonition} Check Your Understanding 
 :class: exercise
 
 Retrieve the box vectors from LAMMPS and print it to the screen.
 
 The command to get the box vectors is `<CELL`. You expect this to contain 9 values (3 for each dimension).
+The data type we expect is an mdi.MDI_DOUBLE.
 
 ```{admonition} Solution
 :class: solution dropdown