diff --git a/chebai/cli.py b/chebai/cli.py
index 392d3225..7b4fb26f 100644
--- a/chebai/cli.py
+++ b/chebai/cli.py
@@ -3,10 +3,11 @@
 from lightning.pytorch.cli import LightningCLI
 from chebai.trainer.InnerCVTrainer import InnerCVTrainer
 
+
 class ChebaiCLI(LightningCLI):
 
     def __init__(self, *args, **kwargs):
-        super().__init__(trainer_class = InnerCVTrainer, *args, **kwargs)
+        super().__init__(trainer_class=InnerCVTrainer, *args, **kwargs)
 
     def add_arguments_to_parser(self, parser):
         for kind in ("train", "val", "test"):
@@ -15,7 +16,7 @@ def add_arguments_to_parser(self, parser):
                     "model.init_args.out_dim",
                     f"model.init_args.{kind}_metrics.init_args.metrics.{average}-f1.init_args.num_labels",
                 )
-        #parser.link_arguments('n_splits', 'data.init_args.inner_k_folds') # doesn't work but I don't know why
+        # parser.link_arguments('n_splits', 'data.init_args.inner_k_folds') # doesn't work but I don't know why
 
     @staticmethod
     def subcommands() -> Dict[str, Set[str]]:
@@ -28,5 +29,6 @@ def subcommands() -> Dict[str, Set[str]]:
             "cv_fit": {"model", "train_dataloaders", "val_dataloaders", "datamodule"},
         }
 
+
 def cli():
     r = ChebaiCLI(save_config_callback=None, parser_kwargs={"parser_mode": "omegaconf"})
diff --git a/chebai/models/electra.py b/chebai/models/electra.py
index cfd7d3d5..12a5ef60 100644
--- a/chebai/models/electra.py
+++ b/chebai/models/electra.py
@@ -24,6 +24,7 @@
 )
 from chebai.preprocessing.reader import MASK_TOKEN_INDEX, CLS_TOKEN
 from chebai.preprocessing.datasets.chebi import extract_class_hierarchy
+from chebai.loss.pretraining import ElectraPreLoss # noqa
 import torch
 import csv
 
@@ -404,3 +405,4 @@ def __call__(self, target, input):
             memberships, target.unsqueeze(-1).expand(-1, -1, 20)
         )
         return loss
+
diff --git a/chebai/preprocessing/datasets/base.py b/chebai/preprocessing/datasets/base.py
index e94495f0..2912881d 100644
--- a/chebai/preprocessing/datasets/base.py
+++ b/chebai/preprocessing/datasets/base.py
@@ -60,11 +60,11 @@ def full_identifier(self):
 
     @property
     def processed_dir(self):
-        return os.path.join("data", self._name, f'chebi_v{self.chebi_version}', "processed", *self.identifier)
+        return os.path.join("data", self._name, "processed", *self.identifier)
 
     @property
     def raw_dir(self):
-        return os.path.join("data", self._name, f'chebi_v{self.chebi_version}', "raw")
+        return os.path.join("data", self._name, "raw")
 
     @property
     def _name(self):
diff --git a/chebai/preprocessing/datasets/chebi.py b/chebai/preprocessing/datasets/chebi.py
index 02c50d16..349a3824 100644
--- a/chebai/preprocessing/datasets/chebi.py
+++ b/chebai/preprocessing/datasets/chebi.py
@@ -15,7 +15,8 @@
 import pickle
 import random
 
-from sklearn.model_selection import train_test_split
+from iterstrat.ml_stratifiers import MultilabelStratifiedShuffleSplit
+
 import fastobo
 import networkx as nx
 import pandas as pd
@@ -117,17 +118,19 @@ def __init__(self, chebi_version_train: int = None, **kwargs):
     def select_classes(self, g, split_name, *args, **kwargs):
         raise NotImplementedError
 
-    def save(self, g, split, split_name: str):
+    def graph_to_raw_dataset(self, g, split_name=None):
+        """Preparation step before creating splits, uses graph created by extract_class_hierarchy()
+        split_name is only relevant, if a separate train_version is set"""
         smiles = nx.get_node_attributes(g, "smiles")
         names = nx.get_node_attributes(g, "name")
 
         print("build labels")
-        print(f"Process {split_name}")
+        print(f"Process graph")
 
         molecules, smiles_list = zip(
             *(
                 (n, smiles)
-                for n, smiles in ((n, smiles.get(n)) for n in split)
+                for n, smiles in ((n, smiles.get(n)) for n in smiles.keys())
                 if smiles
             )
         )
@@ -142,6 +145,10 @@ def save(self, g, split, split_name: str):
         data = pd.DataFrame(data)
         data = data[~data["SMILES"].isnull()]
         data = data[data.iloc[:, 3:].any(axis=1)]
+        return data
+
+    def save(self, data: pd.DataFrame, split_name: str):
+
         pickle.dump(data, open(os.path.join(self.raw_dir, split_name), "wb"))
 
     @staticmethod
@@ -192,44 +199,90 @@ def setup_processed(self):
             self._setup_pruned_test_set()
         self.reader.save_token_cache()
 
-    def get_splits(self, g):
-        fixed_nodes = list(g.nodes)
+    def get_splits(self, df: pd.DataFrame):
         print("Split dataset")
-        random.shuffle(fixed_nodes)
 
-        train_split, test_split = train_test_split(
-            fixed_nodes, train_size=self.train_split, shuffle=True
-        )
+        df_list = df.values.tolist()
+        df_list = [row[3:] for row in df_list]
+
+        msss = MultilabelStratifiedShuffleSplit(n_splits=1, test_size=1-self.train_split, random_state=0)
+
+        train_split = []
+        test_split = []
+        for (train_split, test_split) in msss.split(
+            df_list, df_list,
+        ):
+            train_split = train_split
+            test_split = test_split
+            break
+        df_train = df.iloc[train_split]
+        df_test = df.iloc[test_split]
         if self.use_inner_cross_validation:
-            return train_split, test_split
+            return df_train, df_test
+
+        df_test_list = df_test.values.tolist()
+        df_test_list = [row[3:] for row in df_test_list]
+        validation_split = []
+        test_split = []
+        msss = MultilabelStratifiedShuffleSplit(n_splits=1, test_size=1-self.train_split, random_state=0)
+        for (test_split, validation_split) in msss.split(
+                df_test_list, df_test_list
+        ):
+            test_split = test_split
+            validation_split = validation_split
+            break
 
-        test_split, validation_split = train_test_split(
-            test_split, train_size=self.train_split, shuffle=True
-        )
-        return train_split, test_split, validation_split
+        df_validation = df_test.iloc[validation_split]
+        df_test = df_test.iloc[test_split]
+        return df_train, df_test, df_validation
 
-    def get_splits_given_test(self, g, test_split):
+    def get_splits_given_test(self, df: pd.DataFrame, test_df: pd.DataFrame):
         """ Use test set from another chebi version the model does not train on, avoid overlap"""
-        fixed_nodes = list(g.nodes)
         print(f"Split dataset for chebi_v{self.chebi_version_train}")
-        for node in test_split:
-            if node in fixed_nodes:
-                fixed_nodes.remove(node)
-        random.shuffle(fixed_nodes)
+        df_trainval = df
+        test_smiles = test_df['SMILES'].tolist()
+        mask = []
+        for row in df_trainval:
+            if row['SMILES'] in test_smiles:
+                mask.append(False)
+            else:
+                mask.append(True)
+        df_trainval = df_trainval[mask]
+
         if self.use_inner_cross_validation:
-            return fixed_nodes
+            return df_trainval
+
         # assume that size of validation split should relate to train split as in get_splits()
-        validation_split, train_split = train_test_split(
-            fixed_nodes, train_size=(1 - self.train_split) ** 2, shuffle=True
-        )
-        return train_split, validation_split
+        msss = MultilabelStratifiedShuffleSplit(n_splits=1, test_size=self.train_split ** 2, random_state=0)
+
+        df_trainval_list = df_trainval.tolist()
+        df_trainval_list = [row[3:] for row in df_trainval_list]
+        train_split = []
+        validation_split = []
+        for (train_split, validation_split) in msss.split(
+            df_trainval_list, df_trainval_list
+        ):
+            train_split = train_split
+            validation_split = validation_split
+
+        df_validation = df_trainval.iloc[validation_split]
+        df_train = df_trainval.iloc[train_split]
+        return df_train, df_validation
+
+    @property
+    def processed_dir(self):
+        return os.path.join("data", self._name, f'chebi_v{self.chebi_version}', "processed", *self.identifier)
+
+    @property
+    def raw_dir(self):
+        return os.path.join("data", self._name, f'chebi_v{self.chebi_version}', "raw")
 
     @property
     def processed_file_names_dict(self) -> dict:
         train_v_str = f'_v{self.chebi_version_train}' if self.chebi_version_train else ''
         res = {'test': f"test{train_v_str}.pt"}
         if self.use_inner_cross_validation:
-            res['train_val'] = f'trainval{train_v_str}.pt' # for cv, split train/val on runtime
+            res['train_val'] = f'trainval{train_v_str}.pt'  # for cv, split train/val on runtime
         else:
             res['train'] = f"train{train_v_str}.pt"
             res['validation'] = f"validation{train_v_str}.pt"
@@ -238,10 +291,10 @@ def processed_file_names_dict(self) -> dict:
     @property
     def raw_file_names_dict(self) -> dict:
         train_v_str = f'_v{self.chebi_version_train}' if self.chebi_version_train else ''
-        res = {'test': f"test.pkl"} # no extra raw test version for chebi_version_train - use default test set and only
-                                     # adapt processed file
+        res = {'test': f"test.pkl"}  # no extra raw test version for chebi_version_train - use default test set and only
+        # adapt processed file
         if self.use_inner_cross_validation:
-            res['train_val'] = f'trainval{train_v_str}.pkl' # for cv, split train/val on runtime
+            res['train_val'] = f'trainval{train_v_str}.pkl'  # for cv, split train/val on runtime
         else:
             res['train'] = f"train{train_v_str}.pkl"
             res['validation'] = f"validation{train_v_str}.pkl"
@@ -274,12 +327,13 @@ def prepare_data(self, *args, **kwargs):
                     open(chebi_path, "wb").write(r.content)
                 g = extract_class_hierarchy(chebi_path)
                 splits = {}
+                full_data = self.graph_to_raw_dataset(g)
                 if self.use_inner_cross_validation:
-                    splits['train_val'], splits['test'] = self.get_splits(g)
+                    splits['train_val'], splits['test'] = self.get_splits(full_data)
                 else:
-                    splits['train'], splits['test'], splits['validation'] = self.get_splits(g)
+                    splits['train'], splits['test'], splits['validation'] = self.get_splits(full_data)
                 for label, split in splits.items():
-                    self.save(g, split, self.raw_file_names_dict[label])
+                    self.save(split, self.raw_file_names_dict[label])
             else:
                 # missing test set -> create
                 if not os.path.isfile(os.path.join(self.raw_dir, self.raw_file_names_dict['test'])):
@@ -290,8 +344,9 @@ def prepare_data(self, *args, **kwargs):
                         r = requests.get(url, allow_redirects=True)
                         open(chebi_path, "wb").write(r.content)
                     g = extract_class_hierarchy(chebi_path)
-                    _, test_split, _ = self.get_splits(g)
-                    self.save(g, test_split, self.raw_file_names_dict['test'])
+                    df = self.graph_to_raw_dataset(g, self.raw_file_names_dict['test'])
+                    _, test_split, _ = self.get_splits(df)
+                    self.save(df, self.raw_file_names_dict['test'])
                 else:
                     # load test_split from file
                     with open(os.path.join(self.raw_dir, self.raw_file_names_dict['test']), "rb") as input_file:
@@ -304,12 +359,14 @@ def prepare_data(self, *args, **kwargs):
                     open(chebi_path, "wb").write(r.content)
                 g = extract_class_hierarchy(chebi_path)
                 if self.use_inner_cross_validation:
-                    train_val_data = self.get_splits_given_test(g, test_split)
-                    self.save(g, train_val_data, self.raw_file_names_dict['train_val'])
+                    df = self.graph_to_raw_dataset(g, self.raw_file_names_dict['train_val'])
+                    train_val_df = self.get_splits_given_test(df, test_split)
+                    self.save(train_val_df, self.raw_file_names_dict['train_val'])
                 else:
-                    train_split, val_split = self.get_splits_given_test(g, test_split)
-                    self.save(g, train_split, self.raw_file_names_dict['train'])
-                    self.save(g, val_split, self.raw_file_names_dict['validation'])
+                    df = self.graph_to_raw_dataset(g, self.raw_file_names_dict['train'])
+                    train_split, val_split = self.get_splits_given_test(df, test_split)
+                    self.save(train_split, self.raw_file_names_dict['train'])
+                    self.save(val_split, self.raw_file_names_dict['validation'])
 
 
 class JCIExtendedBase(_ChEBIDataExtractor):
@@ -363,6 +420,7 @@ def select_classes(self, g, split_name, *args, **kwargs):
             fout.writelines(str(node) + "\n" for node in nodes)
         return nodes
 
+
 class ChEBIOverXDeepSMILES(ChEBIOverX):
     READER = dr.DeepChemDataReader
 
@@ -380,6 +438,7 @@ class ChEBIOver50(ChEBIOverX):
     def label_number(self):
         return 1332
 
+
 class ChEBIOver100DeepSMILES(ChEBIOverXDeepSMILES, ChEBIOver100):
     pass
 
diff --git a/chebai/preprocessing/datasets/pubchem.py b/chebai/preprocessing/datasets/pubchem.py
index cb1839b4..b3bb0423 100644
--- a/chebai/preprocessing/datasets/pubchem.py
+++ b/chebai/preprocessing/datasets/pubchem.py
@@ -31,6 +31,8 @@ class PubChem(XYBaseDataModule):
 
     def __init__(self, *args, k=100000, **kwargs):
         self._k = k
+        self.pubchem_url = "https://ftp.ncbi.nlm.nih.gov/pubchem/Compound/Monthly/2023-11-01/Extras/CID-SMILES.gz"
+
         super(PubChem, self).__init__(*args, **kwargs)
 
     @property
@@ -60,18 +62,17 @@ def _load_dict(input_file_path):
                 yield dict(features=smiles, labels=None, ident=ident)
 
     def download(self):
-        url = f"https://ftp.ncbi.nlm.nih.gov/pubchem/Compound/Monthly/2021-10-01/Extras/CID-SMILES.gz"
         if self._k == PubChem.FULL:
             if not os.path.isfile(os.path.join(self.raw_dir, "smiles.txt")):
-                print("Download from", url)
-                r = requests.get(url, allow_redirects=True)
+                print("Download from", self.pubchem_url)
+                r = requests.get(self.pubchem_url, allow_redirects=True)
                 with tempfile.NamedTemporaryFile() as tf:
                     tf.write(r.content)
                     print("Unpacking...")
                     tf.seek(0)
                     with gzip.open(tf, "rb") as f_in:
                         with open(
-                            os.path.join(self.raw_dir, "smiles.txt"), "wb"
+                                os.path.join(self.raw_dir, "smiles.txt"), "wb"
                         ) as f_out:
                             shutil.copyfileobj(f_in, f_out)
         else:
@@ -114,8 +115,8 @@ def processed_file_names(self):
     def prepare_data(self, *args, **kwargs):
         print("Check for raw data in", self.raw_dir)
         if any(
-            not os.path.isfile(os.path.join(self.raw_dir, f))
-            for f in self.raw_file_names
+                not os.path.isfile(os.path.join(self.raw_dir, f))
+                for f in self.raw_file_names
         ):
             print("Downloading data. This may take some time...")
             self.download()
@@ -269,3 +270,6 @@ class PubToxAndChebi100(PubToxAndChebiX):
 
 class PubToxAndChebi50(PubToxAndChebiX):
     CHEBI_X = ChEBIOver50
+
+class PubChemDeepSMILES(PubChem):
+    READER = dr.DeepChemDataReader
diff --git a/chebai/trainer/InnerCVTrainer.py b/chebai/trainer/InnerCVTrainer.py
index ecbe136f..1150d639 100644
--- a/chebai/trainer/InnerCVTrainer.py
+++ b/chebai/trainer/InnerCVTrainer.py
@@ -11,7 +11,7 @@
 from lightning.fabric.plugins.environments import SLURMEnvironment
 from lightning_utilities.core.rank_zero import WarningCache
 
-from sklearn import model_selection
+from iterstrat.ml_stratifiers import MultilabelStratifiedKFold
 from lightning.pytorch.callbacks.model_checkpoint import _is_dir, rank_zero_warn
 
 from chebai.preprocessing.datasets.base import XYBaseDataModule
@@ -28,7 +28,6 @@ def __init__(self, *args, **kwargs):
         # instantiation custom logger connector
         self._logger_connector.on_trainer_init(self.logger, 1)
 
-
     def cv_fit(self, datamodule: XYBaseDataModule, n_splits: int = -1, *args, **kwargs):
         if n_splits < 2:
             self.fit(datamodule=datamodule, *args, **kwargs)
@@ -36,9 +35,10 @@ def cv_fit(self, datamodule: XYBaseDataModule, n_splits: int = -1, *args, **kwar
             datamodule.prepare_data()
             datamodule.setup()
 
-            kfold = model_selection.KFold(n_splits=n_splits)
+            kfold = MultilabelStratifiedKFold(n_splits=n_splits)
 
-            for fold, (train_ids, val_ids) in enumerate(kfold.split(datamodule.train_val_data)):
+            for fold, (train_ids, val_ids) in enumerate(
+                    kfold.split(datamodule.train_val_data, [data['labels'] for data in datamodule.train_val_data])):
                 train_dataloader = datamodule.train_dataloader(ids=train_ids)
                 val_dataloader = datamodule.val_dataloader(ids=val_ids)
                 init_kwargs = self.init_kwargs
@@ -78,7 +78,6 @@ class ModelCheckpointCVSupport(ModelCheckpoint):
     def setup(self, trainer: "Trainer", pl_module: "LightningModule", stage: str) -> None:
         """Same as in parent class, duplicated to be able to call self.__resolve_ckpt_dir"""
         if self.dirpath is not None:
-            print(f'Eliminating existing dirpath {self.dirpath} at ModelCheckpoint setup')
             self.dirpath = None
         dirpath = self.__resolve_ckpt_dir(trainer)
         dirpath = trainer.strategy.broadcast(dirpath)
@@ -126,6 +125,3 @@ def __resolve_ckpt_dir(self, trainer: "Trainer") -> _PATH:
 
         print(f'Now using checkpoint path {ckpt_path}')
         return ckpt_path
-
-
-
diff --git a/configs/data/chebi100_deepSMILES.yml b/configs/data/chebi100_deepSMILES.yml
new file mode 100644
index 00000000..943f0e17
--- /dev/null
+++ b/configs/data/chebi100_deepSMILES.yml
@@ -0,0 +1 @@
+class_path: chebai.preprocessing.datasets.chebi.ChEBIOver100DeepSMILES
\ No newline at end of file
diff --git a/configs/data/pubchem_deepSMILES.yml b/configs/data/pubchem_deepSMILES.yml
new file mode 100644
index 00000000..42f21199
--- /dev/null
+++ b/configs/data/pubchem_deepSMILES.yml
@@ -0,0 +1 @@
+class_path: chebai.preprocessing.datasets.pubchem.PubChemDeepSMILES
diff --git a/configs/loss/electra_pre_loss.yml b/configs/loss/electra_pre_loss.yml
new file mode 100644
index 00000000..06520b2f
--- /dev/null
+++ b/configs/loss/electra_pre_loss.yml
@@ -0,0 +1 @@
+class_path: chebai.loss.pretraining.ElectraPreLoss
diff --git a/configs/model/electra_pretraining.yml b/configs/model/electra_pretraining.yml
new file mode 100644
index 00000000..7b78e48d
--- /dev/null
+++ b/configs/model/electra_pretraining.yml
@@ -0,0 +1,18 @@
+class_path: chebai.models.ElectraPre
+init_args:
+  out_dim: null
+  optimizer_kwargs:
+    lr: 1e-4
+  config:
+    generator:
+      vocab_size: 1400
+      max_position_embeddings: 1800
+      num_attention_heads: 8
+      num_hidden_layers: 6
+      type_vocab_size: 1
+    discriminator:
+      vocab_size: 1400
+      max_position_embeddings: 1800
+      num_attention_heads: 8
+      num_hidden_layers: 6
+      type_vocab_size: 1
\ No newline at end of file
diff --git a/configs/training/default_callbacks.yml b/configs/training/default_callbacks.yml
index 4f27c4d7..7e0eb5fd 100644
--- a/configs/training/default_callbacks.yml
+++ b/configs/training/default_callbacks.yml
@@ -1,6 +1,7 @@
 - class_path: chebai.trainer.InnerCVTrainer.ModelCheckpoint
   init_args:
     monitor: val_micro-f1
+    mode: 'max'
     filename: 'best_{epoch}_{val_loss:.4f}_{val_micro-f1:.2f}'
     every_n_epochs: 1
     save_top_k: 5
diff --git a/configs/training/pretraining_callbacks.yml b/configs/training/pretraining_callbacks.yml
new file mode 100644
index 00000000..d998302d
--- /dev/null
+++ b/configs/training/pretraining_callbacks.yml
@@ -0,0 +1,12 @@
+- class_path: chebai.trainer.InnerCVTrainer.ModelCheckpointCVSupport
+  init_args:
+    monitor: val_loss
+    mode: 'min'
+    filename: 'best_{epoch}_{val_loss:.4f}_{val_micro-f1:.2f}'
+    every_n_epochs: 1
+    save_top_k: 5
+- class_path: chebai.trainer.InnerCVTrainer.ModelCheckpointCVSupport
+  init_args:
+    filename: 'per_{epoch}_{val_loss:.4f}_{val_micro-f1:.2f}'
+    every_n_epochs: 5
+    save_top_k: -1
\ No newline at end of file
diff --git a/configs/training/pretraining_trainer.yml b/configs/training/pretraining_trainer.yml
new file mode 100644
index 00000000..9dd05a3a
--- /dev/null
+++ b/configs/training/pretraining_trainer.yml
@@ -0,0 +1,10 @@
+min_epochs: 100
+max_epochs: 100
+
+default_root_dir: &default_root_dir logs
+logger:
+    class_path: lightning.pytorch.loggers.CSVLogger
+    init_args:
+      save_dir: *default_root_dir
+
+callbacks: pretraining_callbacks.yml
\ No newline at end of file