added hexagonal and goldene

These structures are newly synthesized
2D flat structures.

Signed-off-by: Nick Papior <[email protected]>

CHANGELOG.md

@@ -9,6 +9,8 @@ we hit release version 1.0.0.
 ## [0.15.0] - YYYY-MM-DD
 
 ### Added
+- added the `goldene` 2D lattice, a `hexagonal` Gold 2D structure
+- added the `hexagonal` 2D lattice, close-packed FCC(111) surface
 - improved `atom` projections of states, #750
 - improved typing system
 - `units` to `read_*` for some `Sile`s, #726

docs/api/geom.rst

@@ -77,6 +77,8 @@ Surfaces (slabs)
    honeycomb
    bilayer
    graphene
+   hexagonal
+   goldene
 
 
 Helpers

src/sisl/geom/__init__.py

@@ -53,6 +53,8 @@
   honeycomb
   graphene
   bilayer
+  hexagonal
+  goldene
 
 
 They generally take a lattice-parameter argument, and will all allow

src/sisl/geom/flat.py

@@ -13,7 +13,15 @@
 
 from ._common import geometry_define_nsc
 
-__all__ = ["honeycomb", "graphene", "honeycomb_flake", "graphene_flake", "triangulene"]
+__all__ = [
+    "honeycomb",
+    "graphene",
+    "honeycomb_flake",
+    "graphene_flake",
+    "triangulene",
+    "hexagonal",
+    "goldene",
+]
 
 
 @set_module("sisl.geom")
@@ -65,7 +73,10 @@ def honeycomb(bond: float, atoms: AtomsLike, orthogonal: bool = False) -> Geomet
             atoms,
             lattice=lattice,
         )
+
+    # Set boundary conditions
     geometry_define_nsc(g, [True, True, False])
+
     return g
 
 
@@ -250,3 +261,80 @@ def triangulene(
     geometry_define_nsc(geom, [False, False, False])
 
     return geom
+
+
+def hexagonal(
+    bond: float, atoms: AtomsLike = None, orthogonal: bool = False
+) -> Geometry:
+    """A hexagonal unit-cell with 1 or 2 atoms in the basic unit cell
+
+    The hexagonal unit-cell is the equivalent of an FCC(111) surface, i.e.
+    a close-packed lattice.
+
+    Parameters
+    ----------
+    bond :
+        bond length between atoms (*not* lattice constant)
+    atoms : Atom, optional
+        the atom (or atoms) that the hexagonal lattice consists of.
+    orthogonal :
+        if True returns an orthogonal lattice (2 atoms), otherwise
+        a non-orthogonal lattice (1 atom).
+    """
+    sq3h = 3.0**0.5 * 0.5
+    if orthogonal:
+        lattice = Lattice(
+            np.array([[2 * sq3h, 0.0, 0.0], [0, 1, 0.0], [0.0, 0.0, 10.0]], np.float64)
+            * bond
+        )
+        g = Geometry(
+            np.array(
+                [[0.0, 0.0, 0.0], [sq3h, 0.5, 0.0]],
+                np.float64,
+            )
+            * bond,
+            atoms,
+            lattice=lattice,
+        )
+    else:
+        lattice = Lattice(
+            np.array(
+                [[sq3h, -0.5, 0.0], [sq3h, 0.5, 0.0], [0.0, 0.0, 10.0]], np.float64
+            )
+            * bond
+        )
+        g = Geometry(
+            np.array([[0.0, 0.0, 0.0]], np.float64),
+            atoms,
+            lattice=lattice,
+        )
+
+    # Set boundary conditions
+    geometry_define_nsc(g, [True, True, False])
+
+    return g
+
+
+def goldene(
+    bond: float = 2.7, atoms: AtomsLike = None, orthogonal: bool = False
+) -> Geometry:
+    """A goldene unit-cell with 1 or 2 atoms in the basic unit cell
+
+    Parameters
+    ----------
+    bond :
+        bond length between atoms (*not* lattice constant)
+    atoms : Atom, optional
+        the atom (or atoms) that the hexagonal lattice consists of.
+        Default to Gold atom.
+    orthogonal :
+        if True returns an orthogonal lattice (2 atoms), otherwise
+        a non-orthogonal lattice (1 atom).
+
+    See Also
+    --------
+    hexagonal : the underlying unit cell
+    """
+    if atoms is None:
+        atoms = Atom(Z=79, R=bond * 1.01)
+    return hexagonal(bond, atoms, orthogonal)

src/sisl/geom/tests/test_geom.py

@@ -45,11 +45,14 @@ def test_basic():
     a = rocksalt(5.64, [Atom("Na", R=3), Atom("Cl", R=4)], orthogonal=True)
 
 
-def test_flat():
-    a = graphene()
+@pytest.mark.parametrize(
+    "func, orthogonal",
+    itertools.product([graphene, goldene], [True, False]),
+)
+def test_flat(func, orthogonal):
+    a = func(orthogonal=orthogonal)
     assert is_right_handed(a)
-    graphene(atoms="C")
-    a = graphene(orthogonal=True)
+    a = func(atoms="C", orthogonal=orthogonal)
     assert is_right_handed(a)