Don't use set_calculator for atoms object (#37)

Author: awvwgk

xtb/ase/calculator.py

@@ -33,7 +33,7 @@
 >>> from ase.build import molecule
 >>> from xtb.ase.calculator import XTB
 >>> atoms = molecule('H2O')
->>> atoms.set_calculator(XTB(method="GFN2-xTB"))
+>>> atoms.calc = XTB(method="GFN2-xTB")
 >>> atoms.get_potential_energy()
 -137.9677758730299
 >>> atoms.get_forces()

xtb/ase/test_calculator.py

@@ -199,8 +199,7 @@ def test_gfn1_xtb_3d():
         -0.37178059, -0.37127074,
     ])
 
-    calc = XTB(method="GFN1-xTB")
-    atoms.set_calculator(calc)
+    atoms.calc = XTB(method="GFN1-xTB")
     assert atoms.pbc.all()
 
     assert approx(atoms.get_potential_energy(), abs=thr) == -1256.768167202048
@@ -233,7 +232,7 @@ def test_gfn2_xtb_3d():
     )
 
     calc = XTB(method="GFN2-xTB")
-    atoms.set_calculator(calc)
+    atoms.calc = calc
 
     with raises(CalculationFailed):
         atoms.get_potential_energy()

xtb/ase/test_optimize.py

@@ -100,8 +100,7 @@ def test_gfn2xtb_lbfgs():
         ]),
     )
 
-    calc = XTB(method="GFN2-xTB", solvent="water", accuracy=0.1)
-    atoms.set_calculator(calc)
+    atoms.calc = XTB(method="GFN2-xTB", solvent="water", accuracy=0.1)
     opt = LBFGS(atoms)
     opt.run(fmax=0.1)
 
@@ -142,8 +141,7 @@ def test_gfn2xtb_velocityverlet():
         ]),
     )
 
-    calc = XTB(method="GFN2-xTB", cache_api=False)
-    atoms.set_calculator(calc)
+    atoms.calc = XTB(method="GFN2-xTB", cache_api=False)
 
     dyn = VelocityVerlet(atoms, timestep=1.0*fs)
     dyn.run(20)