Add lattice and Fermi-Hubbard model

.pylintdict

@@ -18,6 +18,8 @@ ao
 april
 arginine
 args
+arrowheads
+arrowstyle
 arxiv
 asparagine
 aspartic
@@ -58,14 +60,17 @@ clbits
 clifford
 cliffords
 cls
+cmap
 cnot
 codec
 coeff
 coeffs
+colormap
 commutativities
 conda
 conf
 config
+connectionstyle
 coord
 coords
 coul
@@ -165,6 +170,7 @@ hilbert
 histidine
 html
 https
+hubbard
 ia
 iajb
 ifdef
@@ -218,6 +224,7 @@ libor
 libxc
 lih
 lijh
+linewidths
 linux
 ljik
 lk
@@ -226,6 +233,7 @@ lookback
 lysine
 macos
 makefile
+matplotlib
 maxdepth
 maxiter
 maxiters
@@ -302,6 +310,7 @@ phenylalanine
 phonons
 polypeptides
 popovas
+pos
 posteriori
 pre
 prebuilt
@@ -338,6 +347,9 @@ rebranding
 refactor
 regs
 repr
+retworkx
+rgb
+rgba
 rhf
 rhs
 robert
@@ -378,6 +390,7 @@ stereochemistry
 sto
 stober
 str
+stylesheet
 subcircuits
 subclasses
 submodules
@@ -437,6 +450,8 @@ verbatim
 vibrational
 vibrationalop
 vibro
+vmax
+vmin
 vqe
 vscf
 vwn

.pylintrc

@@ -34,7 +34,7 @@ unsafe-load-any-extension=no
 # A comma-separated list of package or module names from where C extensions may
 # be loaded. Extensions are loading into the active Python interpreter and may
 # run arbitrary code
-extension-pkg-whitelist=
+extension-pkg-whitelist=retworkx
 
 
 [MESSAGES CONTROL]

docs/apidocs/qiskit_nature.problems.second_quantization.rst

@@ -4,4 +4,3 @@
    :no-members:
    :no-inherited-members:
    :no-special-members:
-

qiskit_nature/problems/__init__.py

@@ -32,5 +32,4 @@
    :toctree:
 
    sampling
-
 """

qiskit_nature/problems/second_quantization/__init__.py

@@ -24,6 +24,14 @@
    BaseProblem
    ElectronicStructureProblem
    VibrationalStructureProblem
+
+Submodules
+==========
+.. autosummary::
+   :toctree: ../stubs/
+
+   lattice
+
 """
 
 from .base_problem import BaseProblem

qiskit_nature/problems/second_quantization/lattice/__init__.py

@@ -0,0 +1,54 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2021.
+#
+# 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.
+
+"""
+Lattice Problems (:mod:`qiskit_nature.problems.second_quantization.lattice`)
+=============================================================================
+The `Lattice` class is used to generate a general lattice,
+which is a graph with complex-valued weights on its edges.
+The `HyperCubicLattice` class and others provide standard lattices with certain
+translational symmetries.
+`Models` include classes that define Hamiltonians on a lattice.
+It takes a lattice object and model parameters as input and
+generates a Hamiltonian defined on the lattice.
+
+.. currentmodule:: qiskit_nature.problems.second_quantization.lattice
+
+Lattice
+==============
+.. autosummary::
+   :toctree: ../stubs/
+
+   Lattice
+   HyperCubicLattice
+   LineLattice
+   SquareLattice
+   TriangularLattice
+
+Models
+=======
+.. autosummary::
+   :toctree: ../stubs/
+
+   FermiHubbardModel
+
+"""
+from .lattice import (
+    BoundaryCondition,
+    LatticeDrawStyle,
+    HyperCubicLattice,
+    Lattice,
+    LineLattice,
+    SquareLattice,
+    TriangularLattice,
+)
+from .models import FermiHubbardModel

qiskit_nature/problems/second_quantization/lattice/lattice/__init__.py

@@ -0,0 +1,19 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2021.
+#
+# 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.
+
+"""A class containing information about the Lattice."""
+from .boundary_condition import BoundaryCondition
+from .hyper_cubic_lattice import HyperCubicLattice
+from .lattice import LatticeDrawStyle, Lattice
+from .line_lattice import LineLattice
+from .square_lattice import SquareLattice
+from .triangular_lattice import TriangularLattice

qiskit_nature/problems/second_quantization/lattice/lattice/boundary_condition.py

@@ -0,0 +1,24 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2021.
+#
+# 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.
+
+"""
+This module declares the available boundary conditions.
+"""
+
+from enum import Enum
+
+
+class BoundaryCondition(Enum):
+    """Boundary condition Enum"""
+
+    OPEN = "open"
+    PERIODIC = "periodic"