diff --git a/pyscf/pbc/dft/multigrid/multigrid_pair.py b/pyscf/pbc/dft/multigrid/multigrid_pair.py
index 6b06199177..7388a80e1d 100644
--- a/pyscf/pbc/dft/multigrid/multigrid_pair.py
+++ b/pyscf/pbc/dft/multigrid/multigrid_pair.py
@@ -83,6 +83,7 @@ class GridLevel_Info(ctypes.Structure):
     '''
     Info about the grid levels.
     '''
+    __slots__ = []
     _fields_ = [("nlevels", ctypes.c_int), # number of grid levels
                 ("rel_cutoff", ctypes.c_double),
                 ("cutoff", ctypes.POINTER(ctypes.c_double)),
@@ -92,6 +93,7 @@ class RS_Grid(ctypes.Structure):
     '''
     Values on real space multigrid.
     '''
+    __slots__ = []
     _fields_ = [("nlevels", ctypes.c_int),
                 ("gridlevel_info", ctypes.POINTER(GridLevel_Info)),
                 ("comp", ctypes.c_int),
@@ -102,6 +104,7 @@ class PGFPair(ctypes.Structure):
     '''
     A primitive Gaussian function pair.
     '''
+    __slots__ = []
     _fields_ = [("ish", ctypes.c_int),
                 ("ipgf", ctypes.c_int),
                 ("jsh", ctypes.c_int),
@@ -114,6 +117,7 @@ class Task(ctypes.Structure):
     '''
     A single task.
     '''
+    __slots__ = []
     _fields_ = [("buf_size", ctypes.c_size_t),
                 ("ntasks", ctypes.c_size_t),
                 ("pgfpairs", ctypes.POINTER(ctypes.POINTER(PGFPair))),
@@ -124,6 +128,7 @@ class TaskList(ctypes.Structure):
     '''
     A task list.
     '''
+    __slots__ = []
     _fields_ = [("nlevels", ctypes.c_int),
                 ("hermi", ctypes.c_int),
                 ("gridlevel_info", ctypes.POINTER(GridLevel_Info)),
diff --git a/pyscf/pbc/gto/_pbcintor.py b/pyscf/pbc/gto/_pbcintor.py
index c5b921b2e0..5b97c2ecd7 100644
--- a/pyscf/pbc/gto/_pbcintor.py
+++ b/pyscf/pbc/gto/_pbcintor.py
@@ -61,6 +61,7 @@ def __del__(self):
             pass
 
 class _CPBCOpt(ctypes.Structure):
+    __slots__ = []
     _fields_ = [('rrcut', ctypes.c_void_p),
                 ('rcut', ctypes.c_void_p),
                 ('fprescreen', ctypes.c_void_p)]
diff --git a/pyscf/pbc/gto/neighborlist.py b/pyscf/pbc/gto/neighborlist.py
index f4a0527ee2..c380011698 100644
--- a/pyscf/pbc/gto/neighborlist.py
+++ b/pyscf/pbc/gto/neighborlist.py
@@ -24,6 +24,7 @@
 libpbc = lib.load_library('libpbc')
 
 class _CNeighborPair(ctypes.Structure):
+    __slots__ = []
     _fields_ = [("nimgs", ctypes.c_int),
                 ("Ls_list", ctypes.POINTER(ctypes.c_int)),
                 ("q_cond", ctypes.POINTER(ctypes.c_double)),
@@ -31,6 +32,7 @@ class _CNeighborPair(ctypes.Structure):
 
 
 class _CNeighborList(ctypes.Structure):
+    __slots__ = []
     _fields_ = [("nish", ctypes.c_int),
                 ("njsh", ctypes.c_int),
                 ("nimgs", ctypes.c_int),
@@ -38,6 +40,7 @@ class _CNeighborList(ctypes.Structure):
 
 
 class _CNeighborListOpt(ctypes.Structure):
+    __slots__ = []
     _fields_ = [("nl", ctypes.POINTER(_CNeighborList)),
                 ('fprescreen', ctypes.c_void_p)]
 
diff --git a/pyscf/scf/_vhf.py b/pyscf/scf/_vhf.py
index 313407a957..78ae6437e2 100644
--- a/pyscf/scf/_vhf.py
+++ b/pyscf/scf/_vhf.py
@@ -300,6 +300,7 @@ def set_dm(self, dm, atm, bas, env):
 
 
 class _CVHFOpt(ctypes.Structure):
+    __slots__ = []
     _fields_ = [('nbas', ctypes.c_int),
                 ('ngrids', ctypes.c_int),
                 ('direct_scf_tol', ctypes.c_double),