From b9bc56d7a3f184f61447c38ce0ae4c4081b467f9 Mon Sep 17 00:00:00 2001
From: Eric Giguere <eric.giguere@calculquebec.ca>
Date: Tue, 23 Apr 2024 14:07:02 -0400
Subject: [PATCH] Fix raise on too large min_step in krylov

---
 qutip/solver/integrator/krylov.py  | 14 ++++++++------
 qutip/tests/solver/test_sesolve.py | 16 +++++++++++++---
 2 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/qutip/solver/integrator/krylov.py b/qutip/solver/integrator/krylov.py
index 5559020982..7e2d9e1d79 100644
--- a/qutip/solver/integrator/krylov.py
+++ b/qutip/solver/integrator/krylov.py
@@ -51,7 +51,7 @@ def _prepare(self):
             krylov_tridiag, krylov_basis = \
                 self._lanczos_algorithm(rand_ket(N).data)
             if (
-                krylov_tridiag.shape[0] < self.options["krylov_dim"]
+                krylov_tridiag.shape[0] < krylov_dim
                 or krylov_tridiag.shape[0] == N
             ):
                 self._max_step = np.inf
@@ -138,20 +138,22 @@ def krylov_error(t):
                 self._compute_psi(t, *reduced_state)
             ) / self.options["atol"])
 
-        dt = self.options["min_step"]
+        # Under 0 will cause an infinite loop in the while loop bellow.
+        dt = max(self.options["min_step"], 1e-14)
+        max_step = max(self.options["max_step"], dt)
         err = krylov_error(dt)
         if err > 0:
-            ValueError(
+            raise ValueError(
                 f"With the krylov dim of {self.options['krylov_dim']}, the "
                 f"error with the minimum step {dt} is {err}, higher than the "
                 f"desired tolerance of {self.options['atol']}."
             )
 
-        while krylov_error(dt * 10) < 0 and dt < self.options["max_step"]:
+        while krylov_error(dt * 10) < 0 and dt < max_step:
             dt *= 10
 
-        if dt > self.options["max_step"]:
-            return self.options["max_step"]
+        if dt > max_step:
+            return max_step
 
         sol = root_scalar(f=krylov_error, bracket=[dt, dt * 10],
                           method="brentq", xtol=self.options['atol'])
diff --git a/qutip/tests/solver/test_sesolve.py b/qutip/tests/solver/test_sesolve.py
index cfa5ee86ea..af247c0aec 100644
--- a/qutip/tests/solver/test_sesolve.py
+++ b/qutip/tests/solver/test_sesolve.py
@@ -301,9 +301,19 @@ def test_krylovsolve(always_compute_step):
     e_op.dims = H.dims
     tlist = np.linspace(0, 1, 11)
     ref = sesolve(H, psi0, tlist, e_ops=[e_op]).expect[0]
-    options = {"always_compute_step", always_compute_step}
-    krylov_sol = krylovsolve(H, psi0, tlist, 20, e_ops=[e_op]).expect[0]
-    np.testing.assert_allclose(ref, krylov_sol)
+    options = {"always_compute_step": always_compute_step}
+    krylov_sol = krylovsolve(H, psi0, tlist, 20, e_ops=[e_op], options=options)
+    np.testing.assert_allclose(ref, krylov_sol.expect[0])
+
+
+def test_krylovsolve_error():
+    H = qutip.rand_herm(256, density=0.2)
+    psi0 = qutip.basis([256], [255])
+    tlist = np.linspace(0, 1, 11)
+    options = {"min_step": 1e10}
+    with pytest.raises(ValueError) as err:
+        krylovsolve(H, psi0, tlist, 20, options=options)
+    assert "error with the minimum step" in str(err.value)
 
 
 def test_feedback():