# qml.qchem.scf¶

scf(mol, n_steps=50, tol=1e-08)[source]

Return a function that performs the self-consistent-field calculations.

In the Hartree-Fock method, molecular orbitals are typically constructed as a linear combination of atomic orbitals

$\phi_i(r) = \sum_{\mu} C_{\mu i} \chi_{\mu}(r),$

with coefficients $$C_{\mu i}$$ that are initially unknown. The self-consistent-field iterations are performed to find a converged set of molecular orbital coefficients that minimize the total energy of the molecular system. This optimization problem can be reduced to solving a linear system of equations which are usually written as

$FC = SCE,$

where $$E$$ is a diagonal matrix of eigenvalues, representing the molecular orbital energies, $$C$$ is the matrix of molecular orbital coefficients, $$S$$ is the overlap matrix and $$F$$ is the Fock matrix, which also depends on the coefficients. Fixing an initial guess $$C_0$$, the corresponding $$F_0$$ is built and the system $$F_0C_0 = SC_0E$$ is solved to obtain a solution $$C_1$$. This process is iteratively repeated until the coefficients are converged.

The key step in in this process is constructing the Fock matrix which is defined as

$F = H + \frac{1}{2} J - K,$

where $$H$$, $$J$$ and $$K$$ are the core Hamiltonian matrix, Coulomb matrix and exchange matrix, respectively. The entries of $$H$$ are computed from the electronic kinetic energy and the electron-nuclear attraction integrals, which are integrals over atomic basis functions. The elements of the $$J$$ and $$K$$ matrices are obtained from the Coulomb and exchange integrals over the basis functions.

Following the procedure in [Lehtola et al. Molecules 2020, 25, 1218], we express the molecular orbital coefficients in terms of a matrix $$X$$ as $$C = X \tilde{C}$$ which gives the following transformed equation

$\tilde{F} \tilde{C} = \tilde{S} \tilde{C} E,$

where $$\tilde{F} = X^T F X$$, $$\tilde{S} = X^T S X$$ and $$S$$ is the overlap matrix. We chose $$X$$ such that $$\tilde{S} = 1$$ as

$X = V \Lambda^{-1/2} V^T,$

where $$V$$ and $$\Lambda$$ are the eigenvectors and eigenvalues of $$S$$, respectively. This gives the eigenvalue equation

$\tilde{F}\tilde{C} = \tilde{C}E,$

which is solved with conventional methods iteratively.

Parameters
• mol (Molecule) – the molecule object

• n_steps (int) – the number of iterations

• tol (float) – convergence tolerance

Returns

function that performs the self-consistent-field calculations

Return type

function

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]
>>> v_fock, coeffs, fock_matrix, h_core, rep_tensor = scf(mol)(*args)
>>> v_fock
array([-0.67578019,  0.94181155])


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