# Source code for pennylane.qchem.hamiltonian

# Copyright 2018-2021 Xanadu Quantum Technologies Inc.

# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at

# Unless required by applicable law or agreed to in writing, software
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
"""
This module contains the functions needed for computing the molecular Hamiltonian.
"""
# pylint: disable= too-many-branches, too-many-arguments, too-many-locals, too-many-nested-blocks
import pennylane as qml

from .hartree_fock import nuclear_energy, scf
from .observable_hf import fermionic_observable, qubit_observable

[docs]def electron_integrals(mol, core=None, active=None):
r"""Return a function that computes the one- and two-electron integrals in the molecular orbital
basis.

The one- and two-electron integrals are required to construct a molecular Hamiltonian in the
second-quantized form

.. math::

H = \sum_{pq} h_{pq} c_p^{\dagger} c_q + \frac{1}{2} \sum_{pqrs} h_{pqrs} c_p^{\dagger} c_q^{\dagger} c_r c_s,

where :math:c^{\dagger} and :math:c are the creation and annihilation operators,
respectively, and :math:h_{pq} and :math:h_{pqrs} are the one- and two-electron integrals.
These integrals can be computed by integrating over molecular orbitals :math:\phi as

.. math::

h_{pq} = \int \phi_p(r)^* \left ( -\frac{\nabla_r^2}{2} - \sum_i \frac{Z_i}{|r-R_i|} \right )  \phi_q(r) dr,

and

.. math::

h_{pqrs} = \int \frac{\phi_p(r_1)^* \phi_q(r_2)^* \phi_r(r_2) \phi_s(r_1)}{|r_1 - r_2|} dr_1 dr_2.

The molecular orbitals are constructed as a linear combination of atomic orbitals as

.. math::

\phi_i = \sum_{\nu}c_{\nu}^i \chi_{\nu}.

The one- and two-electron integrals can be written in the molecular orbital basis as

.. math::

h_{pq} = \sum_{\mu \nu} C_{p \mu} h_{\mu \nu} C_{\nu q},

and

.. math::

h_{pqrs} = \sum_{\mu \nu \rho \sigma} C_{p \mu} C_{q \nu} h_{\mu \nu \rho \sigma} C_{\rho r} C_{\sigma s}.

The :math:h_{\mu \nu} and :math:h_{\mu \nu \rho \sigma} terms refer to the elements of the
core matrix and the electron repulsion tensor, respectively, and :math:C is the molecular
orbital expansion coefficient matrix.

Args:
mol (~qchem.molecule.Molecule): the molecule object
core (list[int]): indices of the core orbitals
active (list[int]): indices of the active orbitals

Returns:
function: function that computes the core constant and the one- and two-electron integrals

**Example**

>>> symbols  = ['H', 'H']
>>> geometry = np.array([[0.0, 0.0, 0.0], [0.0, 0.0, 1.0]], requires_grad = False)
>>> alpha = np.array([[3.42525091, 0.62391373, 0.1688554],
>>> mol = qml.qchem.Molecule(symbols, geometry, alpha=alpha)
>>> args = [alpha]
>>> electron_integrals(mol)(*args)
(1.0,
array([[-1.3902192695e+00,  0.0000000000e+00],
[-4.4408920985e-16, -2.9165331336e-01]]),
array([[[[ 7.1443907755e-01, -2.7755575616e-17],
[ 5.5511151231e-17,  1.7024144301e-01]],
[[ 5.5511151231e-17,  1.7024144301e-01],
[ 7.0185315353e-01,  6.6613381478e-16]]],
[[[-1.3877787808e-16,  7.0185315353e-01],
[ 1.7024144301e-01,  2.2204460493e-16]],
[[ 1.7024144301e-01, -4.4408920985e-16],
[ 6.6613381478e-16,  7.3883668974e-01]]]]))
"""

def _electron_integrals(*args):
r"""Compute the one- and two-electron integrals in the molecular orbital basis.

Args:
*args (array[array[float]]): initial values of the differentiable parameters

Returns:
tuple[array[float]]: 1D tuple containing core constant, one- and two-electron integrals
"""
_, coeffs, _, h_core, repulsion_tensor = scf(mol)(*args)
one = qml.math.einsum("qr,rs,st->qt", coeffs.T, h_core, coeffs)
two = qml.math.swapaxes(
qml.math.einsum(
"ab,cd,bdeg,ef,gh->acfh", coeffs.T, coeffs.T, repulsion_tensor, coeffs, coeffs
),
1,
3,
)
core_constant = nuclear_energy(mol.nuclear_charges, mol.coordinates)(*args)

if core is None and active is None:
return core_constant, one, two

for i in core:
core_constant = core_constant + 2 * one[i][i]
for j in core:
core_constant = core_constant + 2 * two[i][j][j][i] - two[i][j][i][j]

for p in active:
for q in active:
for i in core:
o = qml.math.zeros(one.shape)
o[p, q] = 1.0
one = one + (2 * two[i][p][q][i] - two[i][p][i][q]) * o

one = one[qml.math.ix_(active, active)]
two = two[qml.math.ix_(active, active, active, active)]

return core_constant, one, two

return _electron_integrals

[docs]def fermionic_hamiltonian(mol, cutoff=1.0e-12, core=None, active=None):
r"""Return a function that computes the fermionic Hamiltonian.

Args:
mol (~qchem.molecule.Molecule): the molecule object
cutoff (float): cutoff value for discarding the negligible electronic integrals
core (list[int]): indices of the core orbitals
active (list[int]): indices of the active orbitals

Returns:
function: function that computes the fermionic hamiltonian

**Example**

>>> symbols  = ['H', 'H']
>>> geometry = np.array([[0.0, 0.0, 0.0], [0.0, 0.0, 1.0]], requires_grad = False)
>>> alpha = np.array([[3.42525091, 0.62391373, 0.1688554],
>>> mol = qml.qchem.Molecule(symbols, geometry, alpha=alpha)
>>> args = [alpha]
>>> h = fermionic_hamiltonian(mol)(*args)
"""

def _fermionic_hamiltonian(*args):
r"""Compute the fermionic hamiltonian.

Args:
*args (array[array[float]]): initial values of the differentiable parameters

Returns:
FermiSentence: fermionic Hamiltonian
"""

core_constant, one, two = electron_integrals(mol, core, active)(*args)

return fermionic_observable(core_constant, one, two, cutoff)

return _fermionic_hamiltonian

[docs]def diff_hamiltonian(mol, cutoff=1.0e-12, core=None, active=None):
r"""Return a function that computes the qubit Hamiltonian.

Args:
mol (~qchem.molecule.Molecule): the molecule object
cutoff (float): cutoff value for discarding the negligible electronic integrals
core (list[int]): indices of the core orbitals
active (list[int]): indices of the active orbitals

Returns:
function: function that computes the qubit hamiltonian

**Example**

>>> symbols  = ['H', 'H']
>>> geometry = np.array([[0.0, 0.0, 0.0], [0.0, 0.0, 1.0]], requires_grad = False)
>>> alpha = np.array([[3.42525091, 0.62391373, 0.1688554],
>>> mol = qml.qchem.Molecule(symbols, geometry, alpha=alpha)
>>> args = [alpha]
>>> h = diff_hamiltonian(mol)(*args)
>>> h.coeffs
array([ 0.29817879+0.j,  0.20813365+0.j,  0.20813365+0.j,
0.17860977+0.j,  0.04256036+0.j, -0.04256036+0.j,
-0.04256036+0.j,  0.04256036+0.j, -0.34724873+0.j,
0.13290293+0.j, -0.34724873+0.j,  0.17546329+0.j,
0.17546329+0.j,  0.13290293+0.j,  0.18470917+0.j])
"""

def _molecular_hamiltonian(*args):
r"""Compute the qubit hamiltonian.

Args:
*args (array[array[float]]): initial values of the differentiable parameters

Returns:
Hamiltonian: the qubit Hamiltonian
"""

h_ferm = fermionic_hamiltonian(mol, cutoff, core, active)(*args)

return qubit_observable(h_ferm)

return _molecular_hamiltonian


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