diff --git a/src/pyiron_lammps/compatibility/calculate.py b/src/pyiron_lammps/compatibility/calculate.py
index c77d562..80574a5 100644
--- a/src/pyiron_lammps/compatibility/calculate.py
+++ b/src/pyiron_lammps/compatibility/calculate.py
@@ -385,6 +385,10 @@ def _pressure_to_lammps(pressure, rotation_matrix, units="metal"):
         (float): a single, isotropic pressure to be used with the "iso" option
     """
 
+    # in case no rotation matrix is given, assume identity
+    if rotation_matrix is None:
+        rotation_matrix = np.eye(3)
+
     # If pressure is a scalar, only unit conversion is needed.
     if np.isscalar(pressure):
         return float(pressure) * LAMMPS_UNIT_CONVERSIONS[units]["pressure"]
@@ -403,11 +407,9 @@ def _pressure_to_lammps(pressure, rotation_matrix, units="metal"):
 
     # If necessary, rotate the pressure tensor to the Lammps coordinate frame.
     # Isotropic, hydrostatic pressures are rotation invariant.
-    if (
-        rotation_matrix is not None
-        and not np.isclose(np.matrix.trace(rotation_matrix), 3)
-        and not _is_isotropic_hydrostatic(pressure)
-    ):
+    if not np.isclose(
+        np.matrix.trace(rotation_matrix), 3
+    ) and not _is_isotropic_hydrostatic(pressure):
         if any(p is None for p in pressure):
             raise ValueError(
                 "Cells which are not orthorhombic or an upper-triangular cell are incompatible with Lammps "