diff --git a/ferminet/checkpoint.py b/ferminet/checkpoint.py
index 192fd7a..87f443b 100644
--- a/ferminet/checkpoint.py
+++ b/ferminet/checkpoint.py
@@ -14,12 +14,14 @@
 
 """Super simple checkpoints using numpy."""
 
+import dataclasses
 import datetime
 import os
 from typing import Optional
 import zipfile
 
 from absl import logging
+from ferminet import networks
 import jax
 import jax.numpy as jnp
 import numpy as np
@@ -108,7 +110,7 @@ def save(save_path: str, t: int, data, params, opt_state, mcmc_width) -> str:
     np.savez(
         f,
         t=t,
-        data=data,
+        data=dataclasses.asdict(data),
         params=params,
         opt_state=opt_state,
         mcmc_width=mcmc_width)
@@ -144,16 +146,20 @@ def restore(restore_filename: str, batch_size: Optional[int] = None):
     # Retrieve data from npz file. Non-array variables need to be converted back
     # to natives types using .tolist().
     t = ckpt_data['t'].tolist() + 1  # Return the iterations completed.
-    data = ckpt_data['data']
+    data = networks.FermiNetData(**ckpt_data['data'].item())
     params = ckpt_data['params'].tolist()
     opt_state = ckpt_data['opt_state'].tolist()
     mcmc_width = jnp.array(ckpt_data['mcmc_width'].tolist())
-    if data.shape[0] != jax.device_count():
+    if data.positions.shape[0] != jax.device_count():
       raise ValueError(
-          f'Incorrect number of devices found. Expected {data.shape[0]}, found '
-          f'{jax.device_count()}.')
-    if batch_size and data.shape[0] * data.shape[1] != batch_size:
+          'Incorrect number of devices found. Expected'
+          f' {data.positions.shape[0]}, found {jax.device_count()}.'
+      )
+    if (
+        batch_size
+        and data.positions.shape[0] * data.positions.shape[1] != batch_size
+    ):
       raise ValueError(
           f'Wrong batch size in loaded data. Expected {batch_size}, found '
-          f'{data.shape[0] * data.shape[1]}.')
+          f'{data.positions.shape[0] * data.positions.shape[1]}.')
   return t, data, params, opt_state, mcmc_width
diff --git a/ferminet/train.py b/ferminet/train.py
index 5ebbac0..8b8ea32 100644
--- a/ferminet/train.py
+++ b/ferminet/train.py
@@ -499,6 +499,9 @@ def log_network(*args, **kwargs):
     pos = kfac_jax.utils.broadcast_all_local_devices(pos)
     spins = jnp.reshape(spins, data_shape + spins.shape[1:])
     spins = kfac_jax.utils.broadcast_all_local_devices(spins)
+    data = networks.FermiNetData(
+        positions=pos, spins=spins, atoms=batch_atoms, charges=batch_charges
+    )
 
     t_init = 0
     opt_state_ckpt = None
@@ -523,12 +526,12 @@ def log_network(*args, **kwargs):
         network.orbitals, in_axes=(None, 0, 0, 0, 0), out_axes=0
     )
     sharded_key, subkeys = kfac_jax.utils.p_split(sharded_key)
-    params, pos = pretrain.pretrain_hartree_fock(
+    params, data.positions = pretrain.pretrain_hartree_fock(
         params=params,
-        positions=pos,
+        positions=data.positions,
         spins=pretrain_spins,
-        atoms=batch_atoms,
-        charges=batch_charges,
+        atoms=data.atoms,
+        charges=data.charges,
         batch_network=batch_network,
         batch_orbitals=batch_orbitals,
         network_options=network.options,
@@ -620,13 +623,7 @@ def learning_rate_schedule(t_: jnp.ndarray) -> jnp.ndarray:
         # debug=True
     )
     sharded_key, subkeys = kfac_jax.utils.p_split(sharded_key)
-    opt_state = optimizer.init(
-        params,
-        subkeys,
-        networks.FermiNetData(
-            positions=pos, spins=spins, atoms=batch_atoms, charges=batch_charges
-        ),
-    )
+    opt_state = optimizer.init(params, subkeys, data)
     opt_state = opt_state_ckpt or opt_state  # avoid overwriting ckpted state
   else:
     raise ValueError(f'Not a recognized optimizer: {cfg.optim.optimizer}')
@@ -658,8 +655,6 @@ def learning_rate_schedule(t_: jnp.ndarray) -> jnp.ndarray:
         jnp.asarray(cfg.mcmc.move_width))
   pmoves = np.zeros(cfg.mcmc.adapt_frequency)
 
-  data = networks.FermiNetData(
-      positions=pos, spins=spins, atoms=batch_atoms, charges=batch_charges)
   if t_init == 0:
     logging.info('Burning in MCMC chain for %d steps', cfg.mcmc.burn_in)