max_cycle=0 bug in SOSCF (fix pyscf#2529)

pyscf/soscf/newton_ah.py

@@ -508,11 +508,15 @@ def kernel(mf, mo_coeff=None, mo_occ=None, dm=None,
     # Save mo_coeff and mo_occ because they are needed by function rotate_mo
     mf.mo_coeff, mf.mo_occ = mo_coeff, mo_occ
 
+    scf_conv = False
     e_tot = mf._scf.energy_tot(dm, h1e, vhf)
     fock = mf.get_fock(h1e, s1e, vhf, dm, level_shift_factor=0)
     log.info('Initial guess E= %.15g  |g|= %g', e_tot,
              numpy.linalg.norm(mf._scf.get_grad(mo_coeff, mo_occ, fock)))
 
+    if mf.max_cycle <= 0:
+        return scf_conv, e_tot, mo_energy, mo_coeff, mo_occ
+
     if dump_chk and mf.chkfile:
         chkfile.save_mol(mol, mf.chkfile)
 
@@ -525,7 +529,6 @@ def kernel(mf, mo_coeff=None, mo_occ=None, dm=None,
     next(rotaiter)  # start the iterator
     kftot = jktot = 0
     norm_gorb = 0.
-    scf_conv = False
     cput1 = log.timer('initializing second order scf', *cput0)
 
     for imacro in range(max_cycle):

pyscf/soscf/test/test_newton_ah.py

@@ -143,6 +143,15 @@ def test_nr_uhf(self):
         nr.conv_tol_grad = 1e-5
         self.assertAlmostEqual(nr.kernel(), -75.58051984397145, 9)
 
+        nr1 = scf.newton(mf)
+        nr1.max_cycle = 0 # Should reproduce energy of the initial guess
+        nr1.kernel()
+        self.assertAlmostEqual(nr1.e_tot, mf.e_tot, 12)
+
+        nr1.max_cycle = 2
+        nr1.kernel()
+        self.assertAlmostEqual(nr1.e_tot, nr.e_tot, 12)
+
     def test_nr_uhf_cart(self):
         mol = h2o_z1.copy()
         mol.cart = True