solvers: avoid performing factorization when not needed

kwant/solvers/common.py

@@ -363,6 +363,16 @@ class SparseSolver(object):
                                                   check_hermiticity,
                                                   args, kwargs)
 
+        # Do not perform factorization if no calculation is to be done.
+        len_rhs = sum(i.shape[1] for i in linsys.rhs)
+        if isinstance(linsys.kept_vars, slice):
+            len_kv = len(xrange(*slice.indices(linsys.lhs.shape[0])))
+        else:
+            len_kv = len(linsys.kept_vars)
+        if not(len_rhs and len_kv):
+            return BlockResult(np.zeros((len_kv, len_rhs)),
+                               lead_info, out_leads, in_leads)
+
         flhs = self._factorized(linsys.lhs)
         data = self._solve_linear_sys(flhs, linsys.rhs, linsys.kept_vars)
 
@@ -410,16 +420,18 @@ class SparseSolver(object):
         ldos = np.zeros(num_orb, float)
         factored = None
 
-        factored = self._factorized(h)
-        for mat in rhs:
-            if mat.shape[1] == 0:
-                continue
+        # Do not perform factorization if no further calculation is needed.
+        if sum(i.shape[1] for i in rhs):
+            factored = self._factorized(h)
+            for mat in rhs:
+                if mat.shape[1] == 0:
+                    continue
 
-            for j in xrange(0, mat.shape[1], self.nrhs):
-                jend = min(j + self.nrhs, mat.shape[1])
-                psi = self._solve_linear_sys(factored, [mat[:, j:jend]],
-                                             slice(num_orb))
-                ldos += np.sum(np.square(abs(psi)), axis=1)
+                for j in xrange(0, mat.shape[1], self.nrhs):
+                    jend = min(j + self.nrhs, mat.shape[1])
+                    psi = self._solve_linear_sys(factored, [mat[:, j:jend]],
+                                                 slice(num_orb))
+                    ldos += np.sum(np.square(abs(psi)), axis=1)
 
         return ldos * (0.5 / np.pi)