# dc_dft

Density-corrected DFT.

This command runs a density-corrected DFT calculation, in which an SCF is first performed at a Kohn-Sham DFT or Hartree-Fock level, specified by the first dft subcommand, then followed by a single-shot energy evaluation at another DFT level, specified by the second dft subcommand, using the density obtained from the first SCF calculation.

For example, a DC-DFT calculation using HF for obtaining the converged density and PBE for evaluating the energy might be run using:

dc_dft(
structure( molecule = water )
dft(
xc = HF
ao = '6-31G*'
)
dft(
xc = PBE
ao = '6-31G*'
)
)

This command can appear in the global context.

### Options

ansatz

SCF wavefunction ansatz.

• The type is string
• The default is default
• The value must be one of:
• default - default ansatz for given number of electrons
• r - spin-restricted closed-shell ansatz
• u - spin-unrestricted open-shell ansatz
• ro - spin-restricted open-shell ansatz. By default, the Davidson coupling scheme is used. See rohf_scheme for different choices of the coupling schemes.
• cu -

constrained spin-unrestricted open-shell ansatz of Tsuchimochi and Scuseria.

This ansatz gives the same total energy and wavefunction as the restricted open-shell ansatz ro, but exhibits more robust SCF convergence and gives physically meaningful orbital energies, i.e. satisfying Koopmans' theorem, and the aufbau principle.

Another difference between this ansatz and the ro ansatz is that it yields two different sets of spin orbitals, where as the ro ansatz gives only one set of orbitals for the alpha and beta spins.

However, one can obtain the ro orbitals from the cu wavefunction by setting ro_orbitals_from_cu to true. For example:

hf(
structure( formula = OH bond_length = 1.0 angstrom )
ao = 'Def2-SVP'
ansatz = cu
ro_orbitals_from_cu = true
)


charge

Total molecular charge. No constraint is imposed, but clearly the value has to be such that for a given molecule the number of electrons is nonnegative, and that the number of electrons does not exceed the number that can be described with the given atomic-orbital basis. Note that non-integer values are possible as well.

• The type is real
• The default is 0
check_fock_norm

Check the norm of the Fock matrix against the numerical derivative of the energy before starting SCF. Only for debugging purposes.

• The type is bool
• The default is false
diis

Method of DIIS.

diis_max

Maximum number of vectors for DIIS.

• The type is int
• The default is 10
• The value must be nonnegative
diis_shift

Diagonal shift for regularization of DIIS.

• The type is real
• The default is 0
• The value must be nonnegative
diis_switch_threshold_grad

In DIIS modes ADIIS+CDIIS and EDIIS+CDIIS the algorithm is switched to CDIIS if the gradient falls below this threshold.

• The type is real
• The default is 1e-1
• The value must be nonnegative
energy_threshold

SCF convergence threshold using the absolute value of the energy difference between iterations.

• The type is real
• The default is 1e-6
• The value must be nonnegative
guess

Initial guess for the SCF wavefunction.

• The type is string
• The value must be one of:
• SAD - Superposition of Atomic Densities (SAD).
• H0 - Using eigenvectors of the core Hamiltonian as the initial guess. This is equivalent to setting the initial density to zero.
level_shift

Turn on level shifting to improve SCF convergence. Raises the energy of virtual orbitals by level shifting value. Whether to remove the level shift at the end is controlled by option remove_level_shift.

• The type is real
• The default is 0
• The value must be nonnegative
load

Name of assigned result (containing orbitals) to start the SCF from.

• The type is string
• There is no default value.
max_iter

Maximum number of SCF iterations.

• The type is int
• The default is 128
• The value must be nonnegative
multiplicity

Spin multiplicity $$2S + 1$$ - only valid for integer-electron calculations. For non-integer-electron number, or for higher multiplcities, use the spin option.

By default entos selects the lowest available spin multiplicity.

• The type is string
• There is no default value.
• The value must be one of:
• singlet - 2S + 1 = 1; S = 0
• doublet - 2S + 1 = 2; S = 1/2
• triplet - 2S + 1 = 3; S = 1
• quartet - 2S + 1 = 4; S = 3/2
• quintet - 2S + 1 = 5; S = 2
• sextet - 2S + 1 = 6; S = 5/2
name

Specify the name of a set of results.

This option is deprecated.

• The type is string
• There is no default value.
no_fail

If set to true, the program will continue its execution even if the SCF does not converge. This is useful for frozen-orbital calculations.

• The type is bool
• The default is false
orbital_grad_threshold

SCF convergence threshold using the infinity norm of the orbital gradient.

• The type is real
• The default is 1e-5
• The value must be nonnegative
orthogonalization

Method to orthogonalize the atomic orbital basis. Note that in case of a nearly linearly dependent basis (see overlap_threshold) the canonical algorithm is used irrespective of this option.

• The type is string-lowered
• The default is symmetric
• The value must be one of:
• symmetric - Symmetric orthogonalization of the AO basis. Uses the symmetric inverse square root of the overlap matrix for the orthogonalization.
• canonical - Canonical orthogonalization of the AO basis.
• cholesky - Orthogonalization through Cholesky decomposition of the overlap matrix.
overlap_threshold

Threshold for throwing away the smallest eigenvalues of the overlap matrix. This is a safety measure for calculations with near-linearly dependent basis sets. Usually, the default value should be fine.

• The type is real
• The default is 1e-7
• The value must be nonnegative
print_level

Print level.

• The type is int
• There is no default value.
• The value must be one of:
• -2 - No output
• -1 - Minimum output
• 0 - Output that doesn't scale with system size
• 1 - Output that scales linearly with system size
• 2 - (Debugging) output that scales quadratically with system size
• 3 - (Debugging) output that scales cubically with system size
pseudo_diagonalization

Whether to use pseudo-diagonalization instead of full diagonalization of the Fock matrix. Recommended for calculations on very large systems or with a very large basis.

• The type is bool
• The default is false
remove_level_shift

Remove level shift from final orbitals.

• The type is bool
• The default is true
ro_orbitals_from_cu

Whether to constuct ROHF/ROKS orbitals and orbital energies from a converged CUHF/CUKS wavefunction.

• The type is bool
• The default is true
rohf_scheme

The coupling scheme for constructing the ROHF effective Fock matrix. Note that different choices give the same SCF total energy, but they give different sets of orbitals and orbital energies. See Plakhutin, Gorelik, and Breslavskaya, J. Chem. Phys. 125, 204110 (2006) for a comprehensive discussion on different coupling schemes.

spin

For spin-unrestricted calculations: spin is $$2S$$, where S is the total spin angular momentum

Notice that the value of this option need not be an integer. See the documentation on spin_z for an explanation.

By default entos selects the lowest available spin multiplicity.

• The type is real
• There is no default value.
• The value must be nonnegative
spin_z

For spin-unrestricted calculations, $$2m_s$$, the number of alpha-spin electrons minus the number of beta-spin electrons.

If omitted, spin_z defaults to be the same as the total spin angular momentum.

Notice that the value of this option need not be an integer, and need not be the same as the value of spin. See this paper for an explanation of noninteger charge.

The thinking is similar to thinking about non-integer charge. A fractional-charge (fc) density matrix is a mixed state of the density matrices of two Slater determinants (each with integer charge)

$\rho_\text{fc}(N+1/2) = \rho(N)/2 + \rho(N+1)/2 .$

A fractional-spin (fs) density matrix is a mixed state of two density matrices with different numbers of alpha and beta electrons

$\rho_\text{fs}(S=S, m_s=0) = \rho(N_\alpha = N_\text{el}/2 + S, N_\beta = N_\text{el}/2 - S )/2 + \rho(N_\alpha = N_\text{el}/2 - S, N_\beta = N_\text{el}/2 + S )/2 .$
• The type is real
• There is no default value.
temperature

Specify the temperature for the electrons and perform the SCF calculation in the canonical-ensemble. Convergence is often poor with the default convergence settings, and using the original Pulay DIIS method is recommended (see diis option).

• The type is quantity
• The default is 0 kelvin