Added a README file to explain how to add a driver PES

README.md

@@ -99,7 +99,7 @@ You need to have the `pytest` package installed
 i-pi-test
 ```
 
-See more details in the README file inside the ipi_tests folder.
+See more details in the README file inside the `ipi_tests` folder.
 
 Contributing
 ------------

drivers/py/README.md

@@ -0,0 +1,27 @@
+i-PI driver - Python version
+============================
+
+This is a Python version of a simple driver code that computes energy and forces and communicates 
+them with i-PI using TCP/IP or Unix sockets. 
+Run `i-pi-py_driver -h` to see a list of options and a brief explanation. 
+
+Adding a new PES
+----------------
+
+Besides a couple of demonstrative potential energy evaluators, this driver is useful to add
+potential-energy surface calculator that facilitate interfacing from any Python-based potential. 
+
+Adding a new PES to the driver is easy: take `pes/dummy.py` as a template and add a new 
+file to the `pes/` folder. You can look at some of the other drivers to understand how to
+process the command line parameters passed through the `-o` option to the driver.
+
+It is important to set the following global variables in the new file:
+
+```python
+__DRIVER_NAME__ = "pes"
+__DRIVER_CLASS__ = "PES_class"
+```
+
+`__DRIVER_NAME__` is the name that will be used to identify the pes in the `-m` option.
+`__DRIVER_CLASS__` is the name you use for the class contained in the new PES file.
+

drivers/py/pes/pet.py

@@ -61,7 +61,12 @@ def check_arguments(self):
         )
 
     def __call__(self, cell, pos):
-        """Get energies, forces, and stresses from the PET model"""
+        """Get energies, forces, and stresses from the MACE model
+        This routine assumes that the client will take positions
+        in angstrom, and return energies in electronvolt, and forces
+        in ev/ang.
+        """
+
         pos_pet = unit_to_user("length", "angstrom", pos)
         # librascal expects ASE-format, cell-vectors-as-rows
         cell_pet = unit_to_user("length", "angstrom", cell.T)