Npt implementation fixes in unit conversion (duartegroup#120)

Change reporting the pressure to ensure it is being done in bars and not atomic units and remove redundant functions for the conversion of pressure and compressibility to atomic units.

mlptrain/sampling/md.py

@@ -50,6 +50,9 @@ def run_mlp_md(
     directory. Note that NPT simulations are currently only implemented in
     production runs and not in active learning.
 
+    directory. Note that NPT simulations are currently only implemented in
+    production runs and not in active learning.
+
 
     ---------------------------------------------------------------------------
     Arguments:
@@ -365,14 +368,14 @@ def _run_dynamics(
 
     if all([value is not None for value in [pressure, compress]]) and temp > 0:
         # Run NPT dynamics if pressure and compressibility are specified
-        pressure = convert_pressure_to_ase_units(pressure)
-        compress = convert_compressibility_to_ase_units(compress)
+        pressure_au = pressure * ase_units.bar
+        compress_au = compress / ase_units.bar
         dyn = NPTBerendsen(
             ase_atoms,
             dt_ase,
             temperature_K=temp,
-            pressure_au=pressure,
-            compressibility_au=compress,
+            pressure_au=pressure_au,
+            compressibility_au=compress_au,
         )
         logger.info(
             f'Initialising NPT Berendsen dynamics at {pressure} bar and {temp} K'
@@ -704,21 +707,3 @@ def _remove_colvar_duplicate_frames(
                 f.write(line)
 
     return None
-
-
-def convert_pressure_to_ase_units(
-    pressure: float,
-) -> float:
-    """
-    Converts pressure given in bar to ase units of eV/A^3
-    """
-    return pressure * 0.000006241509
-
-
-def convert_compressibility_to_ase_units(
-    compressibility: float,
-) -> float:
-    """
-    Converts pressure given in bar^-1 to ase units of A^3/eV
-    """
-    return compressibility * 160217.66531138544