Fixed NaCl minimal example for pretrained model

Original NaCl example code was not working properly, follow this on github https://github.com/ACEsuit/mace/issues/261. It can be easily fixed by changing ASE NPT to ASE Langevin and it is also fast and light anyways.

README.md

@@ -24,7 +24,8 @@ pip install git+https://github.com/ACEsuit/mace.git
 ```python
 from mace.calculators import MACECalculator
 from ase import Atoms
-from ase.md.npt import NPT
+from ase.md.langevin import Langevin
+from ase import units
 
 atoms = Atoms("NaCl")
 
@@ -36,14 +37,18 @@ calculator = MACECalculator(
 
 atoms.calc = calculator
 
-npt = NPT(
-  atoms=atoms,
-  timestep=timestep,
-  temperature_K=temperature,
-  externalstress=externalstress,
-)
+## Suggested for test ##
+timestep = 0.5 * units.fs
+temperature = 300 #Kelvins
+friction = 0.1
+
+steps = 10
+
+dyn = Langevin(atoms, timestep,  temperature_K=temperature, friction=friction,
+               fixcm=False)
+
+dyn.run(steps)
 
-npt.run(steps)
 ```
 
 # Citing