Print b3lyp warning when needed (issue pyscf#2029) (pyscf#2040)

* Hide b3lyp warning (issue pyscf#2029)

* Backward compatibility for xcfun

* typo

pyscf/dft/libxc.py

@@ -767,14 +767,7 @@ def _xc_key_without_underscore(xc_keys):
     'TPSS0'         : '.25*HF + .75*TPSS, TPSS',
 })  # noqa: E501
 
-# Issue 1480
-if not hasattr(__config__, 'B3LYP_WITH_VWN5'):
-    warnings.warn('Since PySCF-2.3, B3LYP (and B3P86) are changed to the VWN-RPA variant, '
-                  'corresponding to the original definition by Stephens et al. (issue 1480) '
-                  'and the same as the B3LYP functional in Gaussian. '
-                  'To restore the VWN5 definition, you can put the setting '
-                  '"B3LYP_WITH_VWN5 = True" in pyscf_conf.py')
-elif getattr(__config__, 'B3LYP_WITH_VWN5', False):
+if getattr(__config__, 'B3LYP_WITH_VWN5', False):
     XC_CODES['B3P86' ] = 'B3P86V5'
     XC_CODES['B3LYP' ] = 'B3LYP5'
     XC_CODES['X3LYP' ] = 'X3LYP5'
@@ -1102,6 +1095,16 @@ def parse_xc(description):
     elif not isinstance(description, str): #isinstance(description, (tuple,list)):
         return parse_xc('%s,%s' % tuple(description))
 
+    if (description.upper() in ('B3P86', 'B3LYP', 'X3LYP') and
+        not getattr(parse_xc, 'b3lyp5_warned', False) and
+        not hasattr(__config__, 'B3LYP_WITH_VWN5')):
+        parse_xc.b3lyp5_warned = True
+        warnings.warn('Since PySCF-2.3, B3LYP (and B3P86) are changed to the VWN-RPA variant, '
+                      'corresponding to the original definition by Stephens et al. (issue 1480) '
+                      'and the same as the B3LYP functional in Gaussian. '
+                      'To restore the VWN5 definition, you can put the setting '
+                      '"B3LYP_WITH_VWN5 = True" in pyscf_conf.py')
+
     def assign_omega(omega, hyb_or_sr, lr=0):
         if hyb[2] == omega or omega == 0:
             hyb[0] += hyb_or_sr

pyscf/dft/xcfun.py

@@ -252,7 +252,12 @@
           'B97XC'   , 'B97_1XC' , 'B97_2XC' , 'M05XC'   , 'TPSSH'   ,
           'HFLYP'}
 RSH_XC = {'CAMB3LYP'}
-MAX_DERIV_ORDER = ctypes.c_int.in_dll(_itrf, 'XCFUN_max_deriv_order').value
+
+# The compatibility with the old libxcfun_itrf.so library
+try:
+    MAX_DERIV_ORDER = ctypes.c_int.in_dll(_itrf, 'XCFUN_max_deriv_order').value
+except ValueError:
+    MAX_DERIV_ORDER = 3
 
 VV10_XC = {
     'B97M_V'    : (6.0, 0.01),