Add HexagonalLattice Class

qiskit_nature/second_q/hamiltonians/lattices/__init__.py

@@ -24,6 +24,7 @@
    SquareLattice
    TriangularLattice
    HyperCubicLattice
+   HexagonalLattice
 
 """
 
@@ -33,6 +34,7 @@
 from .line_lattice import LineLattice
 from .square_lattice import SquareLattice
 from .triangular_lattice import TriangularLattice
+from .hexagonal_lattice import HexagonalLattice
 
 __all__ = [
     "BoundaryCondition",
@@ -42,4 +44,5 @@
     "SquareLattice",
     "TriangularLattice",
     "HyperCubicLattice",
+    "HexagonalLattice",
 ]

qiskit_nature/second_q/hamiltonians/lattices/hexagonal_lattice.py

@@ -0,0 +1,73 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2021, 2022.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""The hexagonal lattice"""
+from rustworkx import generators
+
+from .lattice import Lattice
+
+
+class HexagonalLattice(Lattice):
+    """Hexagonal lattice."""
+
+    def __init__(
+        self,
+        rows: int,
+        cols: int,
+        edge_parameter: complex = 1.0,
+        onsite_parameter: complex = 0.0,
+    ) -> None:
+        """
+        Args:
+            rows: Length of the x direction.
+            cols: Length of the y direction.
+            edge_parameter: Weight on all the edges, specified as a single value.
+                Defaults to 1.0.
+            onsite_parameter: Weight on the self-loops, which are edges connecting a node to itself.
+                Defaults to 0.0.
+        """
+        self._rows = rows
+        self._cols = cols
+        self._edge_parameter = edge_parameter
+        self._onsite_parameter = onsite_parameter
+
+        graph = generators.hexagonal_lattice_graph(rows, cols, multigraph=False)
+
+        edges = graph.weighted_edge_list()
+
+        # Add edge weights
+        for idx, _ in enumerate(edges):
+            graph.update_edge_by_index(idx, self._edge_parameter)
+
+        # Add self loops
+        for node in graph.node_indices():
+            graph.add_edges_from([(node, node, self._onsite_parameter)])
+
+        super().__init__(graph)
+
+    @property
+    def edge_parameter(self) -> complex:
+        """Weights on all edges.
+
+        Returns:
+            the parameter for the edges.
+        """
+        return self._edge_parameter
+
+    @property
+    def onsite_parameter(self) -> complex:
+        """Weight on the self-loops (edges connecting a node to itself).
+
+        Returns:
+            the parameter for the self-loops.
+        """
+        return self._onsite_parameter