diff --git a/mlff/cAPI/mlff_structure_relaxation.py b/mlff/cAPI/mlff_structure_relaxation.py
index 6c581ea..45dbf3b 100644
--- a/mlff/cAPI/mlff_structure_relaxation.py
+++ b/mlff/cAPI/mlff_structure_relaxation.py
@@ -1,4 +1,5 @@
 import argparse
+import numpy as np
 import os
 import logging
 
@@ -57,6 +58,8 @@ def run_relaxation():
     parser.add_argument('--qn_tol', type=float, required=False, default=1e-4)
     parser.add_argument('--qn_max_steps', type=int, required=False, default=200)
 
+    parser.add_argument('--optimizer', type=str, required=False, default='QuasiNewton')
+
     parser.add_argument('--mic', type=str, required=False, default=None,
                         help='Minimal image convention.')
 
@@ -221,12 +224,14 @@ def load_start_geometry(f: str) -> Atoms:
         #
         # scales = read_json(os.path.join(ckpt_dir, 'scales.json'))
 
-        potential = mdx.MLFFPotential.create_from_ckpt_dir(ckpt_dir=ckpt_dir, dtype=_mdx_dtype)
-        calc = mlffCalculator(potential=potential,
-                              capacity_multiplier=1.25,
-                              F_to_eV_Ang=default_access(conversion_table, key=F_key, default=eV),
-                              E_to_eV=default_access(conversion_table, key=E_key, default=eV),
-                              )
+        calc = mlffCalculator.create_from_ckpt_dir(
+            ckpt_dir=ckpt_dir,
+            capacity_multiplier=1.25,
+            add_energy_shift=False,
+            F_to_eV_Ang=default_access(conversion_table, key=F_key, default=eV),
+            E_to_eV=default_access(conversion_table, key=E_key, default=eV),
+            dtype=np.float64,
+        )
 
         molecule.set_calculator(calc)
 
@@ -234,7 +239,7 @@ def load_start_geometry(f: str) -> Atoms:
         from ase.io import write
         write(os.path.join(save_dir, 'init_structure.xyz'), molecule)
         # do a geometry relaxation
-        qn = ase_opt.LBFGS(molecule)
+        qn = getattr(ase_opt, args.optimizer)(molecule)
         converged = qn.run(qn_tol, qn_max_steps)
         if converged:
             write(os.path.join(save_dir, 'relaxed_structure.xyz'), molecule)
diff --git a/mlff/md/calculator.py b/mlff/md/calculator.py
index 56a4345..a0637fe 100644
--- a/mlff/md/calculator.py
+++ b/mlff/md/calculator.py
@@ -37,7 +37,7 @@ def create_from_ckpt_dir(cls,
                              F_to_eV_Ang: float = 1.,
                              capacity_multiplier: float = 1.25,
                              add_energy_shift: bool = False,
-                             dtype: np.dtype = np.float32):
+                             dtype: np.dtype = np.float64):
 
         mlff_potential = MLFFPotential.create_from_ckpt_dir(
             ckpt_dir=ckpt_dir,
@@ -60,7 +60,7 @@ def __init__(
             F_to_eV_Ang: float = 1.,
             capacity_multiplier: float = 1.25,
             calculate_stress: bool = False,
-            dtype: np.dtype = np.float32,
+            dtype: np.dtype = np.float64,
             *args,
             **kwargs
     ):
@@ -144,7 +144,7 @@ def calculate(self, atoms=None, *args, **kwargs):
 
         output = self.calculate_fn(System(R=R, Z=z, cell=cell), neighbors=neighbors)  # note different cell convention
 
-        self.results = jax.tree_map(lambda x: np.array(x), output)
+        self.results = jax.tree_map(lambda x: np.array(x, dtype=self.dtype), output)
 
 
 def to_displacement(cell):