[WIP] SAD guess PTs

include/simde/density/density.hpp

@@ -17,3 +17,4 @@
 #pragma once
 #include "simde/density/mp2_density.hpp"
 #include "simde/density/scf_density.hpp"
+#include "simde/density/scf_density_fractional.hpp"

include/simde/density/scf_density_fractional.hpp

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2022 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/types.hpp"
+#include <pluginplay/pluginplay.hpp>
+
+namespace simde {
+
+/** @brief API for modules which create densities from orbitals which can have
+ * fractional occupations.
+ *
+ *  Modules which satisfy the `SCFDensityFractional` property type effectively
+ *  have an API: `simde::type::el_density (const canonical_space&,
+ * std::vector<double>&)`. For example, the `SCF::DensityFractional` will
+ * compute the density by contracting the occupied orbitals together with their
+ * occupations.
+ */
+DECLARE_DERIVED_PROPERTY_TYPE(SCFDensityFractional, SCFDensity);
+
+PROPERTY_TYPE_INPUTS(SCFDensityFractional) {
+    auto rv =
+      pluginplay::declare_input().add_field<const std::vector<double>&>("occ");
+    rv["occ"].set_description("vector of orbital occupations");
+    return rv;
+}
+
+PROPERTY_TYPE_RESULTS(SCFDensityFractional) {
+    return pluginplay::declare_result();
+}
+
+} // namespace simde