Adds a module for Born-Oppenheimer approximation

src/cxx/nux/bo_approximation.cpp

@@ -0,0 +1,89 @@
+/*
+ * Copyright 2024 NWChemEx-Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "nux_modules.hpp"
+
+namespace nux {
+
+using simde::type::chemical_system;
+using simde::type::hamiltonian;
+using simde::type::many_electrons;
+using pt = simde::Convert<hamiltonian, chemical_system>;
+
+const auto desc = R"(
+Born-Oppenheimer Hamiltonian Driver
+-----------------------------------
+
+This module defines the algorithm used throughout NWChemEx to convert a 
+ChemicalSystem object into a Hamiltonian object using the Born-Oppenheimer
+approximation.
+
+Specifically this driver will convert:
+- ManyElectrons to a T_e and V_ee terms.
+- Nuclei to a V_nn
+- ManyElectrons + Nuclei to a V_en term.
+
+The terms will be added following the "usual" ordering:
+
+- Electronic Hamiltonian
+   - Electronic kinetic energy
+   - Electron-nucleus attraction
+   - Electron-electron repulsion
+- Nuclear-nuclear repulsion
+
+Terms that are zero will not appear in the Hamiltonian.
+)";
+
+MODULE_CTOR(BOApproximation) {
+  satisfies_property_type<pt>();
+  description(desc);
+}
+
+MODULE_RUN(BOApproximation) {
+  const auto &[sys] = pt::unwrap_inputs(inputs);
+
+  // Step 0: Decompose system into pieces
+  const auto electrons = sys.molecule().electrons();
+  const auto nuclei = sys.molecule().nuclei().as_nuclei();
+  bool has_electrons = electrons.size();
+  bool has_nuclei = nuclei.size();
+
+  using simde::type::T_e_type;
+  using simde::type::V_ee_type;
+  using simde::type::V_en_type;
+  using simde::type::V_nn_type;
+
+  // Step 1: Create non-zero terms and add them to the Hamiltonian
+  // N.b. the logic is a bit convoluted to conform to "usual" ordering (see
+  // module description)
+
+  hamiltonian H;
+  if (has_electrons) {
+    H.emplace_back(1.0, std::make_unique<T_e_type>(electrons));
+    if (has_nuclei)
+      H.emplace_back(1.0, std::make_unique<V_en_type>(electrons, nuclei));
+    if (electrons.size() > 1)
+      H.emplace_back(1.0, std::make_unique<V_ee_type>(electrons, electrons));
+  }
+  if (nuclei.size() > 1) {
+    H.emplace_back(1.0, std::make_unique<V_nn_type>(nuclei, nuclei));
+  }
+
+  auto rv = results();
+  return pt::wrap_results(rv, H);
+}
+
+} // namespace nux

src/cxx/nux/nux.cpp

@@ -14,10 +14,13 @@
  * limitations under the License.
  */
 
+#include "nux_modules.hpp"
 #include <nux/nux.hpp>
 
 namespace nux {
 
-void load_modules(pluginplay::ModuleManager &mm) {}
+void load_modules(pluginplay::ModuleManager &mm) {
+  mm.add_module<BOApproximation>("Born-Oppenheimer Approximation");
+}
 
 } // namespace nux

src/cxx/nux/nux_modules.hpp

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2024 NWChemEx-Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+#include <simde/simde.hpp>
+
+namespace nux {
+
+DECLARE_MODULE(BOApproximation);
+
+}

tests/cxx/test_nux.hpp

@@ -17,5 +17,30 @@
 #include <catch2/catch_approx.hpp>
 #include <catch2/catch_template_test_macros.hpp>
 #include <catch2/catch_test_macros.hpp>
+#include <simde/simde.hpp>
 
-namespace test_nux {}
+namespace test_nux {
+
+inline auto h_nucleus(double x, double y, double z) {
+  return simde::type::nucleus("H", 1ul, 1836.15, x, y, z);
+}
+
+template <typename ResultType> ResultType make_h2() {
+  using simde::type::chemical_system;
+  using simde::type::molecule;
+  using simde::type::nuclei;
+  if constexpr (std::is_same_v<ResultType, nuclei>) {
+    auto h0 = h_nucleus(0.0, 0.0, 0.0);
+    auto h1 = h_nucleus(0.0, 0.0, 1.3984);
+    return nuclei{h0, h1};
+  } else if constexpr (std::is_same_v<ResultType, molecule>) {
+    return molecule(0, 1, make_h2<nuclei>());
+  } else if constexpr (std::is_same_v<ResultType, chemical_system>) {
+    return chemical_system(make_h2<molecule>());
+  } else {
+    // We know this assert fails if we're in the else statement
+    // Getting here means you provided a bad type.
+    static_assert(std::is_same_v<ResultType, nuclei>);
+  }
+}
+} // namespace test_nux

tests/cxx/unit_tests/bo_approximation.cpp

@@ -0,0 +1,97 @@
+/*
+ * Copyright 2024 NWChemEx-Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "../test_nux.hpp"
+#include <nux/nux.hpp>
+
+using namespace nux;
+
+TEST_CASE("BOApproximation") {
+  pluginplay::ModuleManager mm;
+  load_modules(mm);
+
+  using simde::type::chemical_system;
+  using simde::type::hamiltonian;
+  using simde::type::T_e_type;
+  using simde::type::V_ee_type;
+  using simde::type::V_en_type;
+  using simde::type::V_nn_type;
+
+  using pt = simde::Convert<hamiltonian, chemical_system>;
+
+  auto &mod = mm.at("Born-Oppenheimer Approximation");
+
+  SECTION("Empty chemical system") {
+    chemical_system sys;
+    auto H = mod.run_as<pt>(sys);
+    REQUIRE(H == hamiltonian{});
+  }
+
+  SECTION("H2") {
+    auto h2 = test_nux::make_h2<chemical_system>();
+    auto electrons = h2.molecule().electrons();
+    auto nuclei = h2.molecule().nuclei().as_nuclei();
+
+    auto H = mod.run_as<pt>(h2);
+    hamiltonian H_corr;
+    H_corr.emplace_back(1.0, std::make_unique<T_e_type>(electrons));
+    H_corr.emplace_back(1.0, std::make_unique<V_en_type>(electrons, nuclei));
+    H_corr.emplace_back(1.0, std::make_unique<V_ee_type>(electrons, electrons));
+    H_corr.emplace_back(1.0, std::make_unique<V_nn_type>(nuclei, nuclei));
+    REQUIRE(H == H_corr);
+  }
+
+  SECTION("H atom") {
+    simde::type::nuclei nuclei{test_nux::h_nucleus(0.0, 0.0, 0.0)};
+    simde::type::chemical_system h(simde::type::molecule(0, 2, nuclei));
+
+    auto electrons = h.molecule().electrons();
+
+    auto H = mod.run_as<pt>(h);
+    hamiltonian H_corr;
+    H_corr.emplace_back(1.0, std::make_unique<T_e_type>(electrons));
+    H_corr.emplace_back(1.0, std::make_unique<V_en_type>(electrons, nuclei));
+    REQUIRE(H == H_corr);
+  }
+
+  SECTION("H anion") {
+    simde::type::nuclei nuclei{test_nux::h_nucleus(0.0, 0.0, 0.0)};
+    simde::type::chemical_system h(simde::type::molecule(-1, 1, nuclei));
+
+    auto electrons = h.molecule().electrons();
+
+    auto H = mod.run_as<pt>(h);
+    hamiltonian H_corr;
+    H_corr.emplace_back(1.0, std::make_unique<T_e_type>(electrons));
+    H_corr.emplace_back(1.0, std::make_unique<V_en_type>(electrons, nuclei));
+    H_corr.emplace_back(1.0, std::make_unique<V_ee_type>(electrons, electrons));
+    REQUIRE(H == H_corr);
+  }
+
+  SECTION("H2 cation") {
+    auto nuclei = test_nux::make_h2<simde::type::nuclei>();
+    simde::type::chemical_system h(simde::type::molecule(1, 2, nuclei));
+
+    auto electrons = h.molecule().electrons();
+
+    auto H = mod.run_as<pt>(h);
+    hamiltonian H_corr;
+    H_corr.emplace_back(1.0, std::make_unique<T_e_type>(electrons));
+    H_corr.emplace_back(1.0, std::make_unique<V_en_type>(electrons, nuclei));
+    H_corr.emplace_back(1.0, std::make_unique<V_nn_type>(nuclei, nuclei));
+    REQUIRE(H == H_corr);
+  }
+}

tests/cxx/unit_tests/nux.cpp

@@ -16,7 +16,9 @@
 
 #include "../test_nux.hpp"
 #include <nux/nux.hpp>
+
 TEST_CASE("load_plugin") {
   pluginplay::ModuleManager mm;
   nux::load_modules(mm);
+  REQUIRE(mm.size() > 0);
 }

tests/python/integration_tests/test_nux/test_plugin.py

@@ -19,6 +19,9 @@
 
 class TestLoadModules(unittest.TestCase):
 
+    def test_load_modules(self):
+        self.assertGreater(self.mm.size(), 0)
+
     def setUp(self):
         self.mm = ModuleManager()
         nux.load_modules(self.mm)

tests/python/test_nux.py

@@ -0,0 +1,26 @@
+# Copyright 2024 NWChemEx-Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import chemist
+
+
+def h_nucleus(x, y, z):
+    return chemist.Nucleus('H', 1, 1836.15, x, y, z)
+
+
+def h2_nuclei():
+    h0 = h_nucleus(0.0, 0.0, 0.0)
+    h1 = h_nucleus(0.0, 0.0, 1.3984)
+
+    return chemist.Nuclei(h0, h1)

tests/python/unit_tests/test_nux/test_bo_approximation.py

@@ -0,0 +1,34 @@
+# Copyright 2024 NWChemEx-Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from pluginplay import ModuleManager
+import nux
+import simde
+import chemist
+import unittest
+
+
+class TestBOApproximation(unittest.TestCase):
+
+    def test_empty_chemical_system(self):
+        sys = chemist.ChemicalSystem()
+        # Hamiltonian isn't exposed to Python yet...
+        #H = self.mm.run_as(self.pt, 'Born-Oppenheimer Approximation', sys)
+        #H_corr = chemist.qm_operator.Hamiltonian()
+        #self.assertEqual(H, H_corr)
+
+    def setUp(self):
+        self.mm = ModuleManager()
+        nux.load_modules(self.mm)
+        self.pt = simde.HamiltonianFromChemicalSystem()

tests/python/unit_tests/test_nux/test_plugin.py

@@ -19,6 +19,9 @@
 
 class TestLoadModules(unittest.TestCase):
 
+    def test_load_modules(self):
+        self.assertGreater(self.mm.size(), 0)
+
     def setUp(self):
         self.mm = ModuleManager()
         nux.load_modules(self.mm)