diff --git a/dpdata/plugins/openmx.py b/dpdata/plugins/openmx.py
index 930990ee..f340857c 100644
--- a/dpdata/plugins/openmx.py
+++ b/dpdata/plugins/openmx.py
@@ -5,8 +5,31 @@
 
 @Format.register("openmx")
 class OPENMXFormat(Format):
+    """Format for the `OpenMX <https://www.openmx-square.org/>`.
+
+    OpenMX (Open source package for Material eXplorer) is a nano-scale material simulation package based on DFT, norm-conserving pseudopotentials, and pseudo-atomic localized basis functions.
+
+    Note that two output files, System.Name.dat and System.Name.md, are required.
+
+    Use the `openmx/out` keyword argument to supply this format.
+    """
+
     @Format.post("rot_lower_triangular")
-    def from_system(self, file_name, begin=0, step=1, **kwargs):
+    def from_system(self, file_name: str, **kwargs) -> dict:
+        """Read from OpenMX output.
+
+        Parameters
+        ----------
+        file_name : str
+            file name, which is specified by a input file, i.e. System.Name.dat
+        **kwargs : dict
+            other parameters
+
+        Returns
+        -------
+        dict
+            data dict
+        """
         fname = f"{file_name}.dat"
         mdname = f"{file_name}.md"
 
@@ -18,7 +41,21 @@ def from_system(self, file_name, begin=0, step=1, **kwargs):
         return data
 
     @Format.post("rot_lower_triangular")
-    def from_labeled_system(self, file_name, begin=0, step=1, **kwargs):
+    def from_labeled_system(self, file_name: str, **kwargs) -> dict:
+        """Read from OpenMX output.
+
+        Parameters
+        ----------
+        file_name : str
+            file name, which is specified by a input file, i.e. System.Name.dat
+        **kwargs : dict
+            other parameters
+
+        Returns
+        -------
+        dict
+            data dict
+        """
         fname = f"{file_name}.dat"
         mdname = f"{file_name}.md"