qrunch.chemistry.ground_state_problem.calculators.coupled_cluster_ground_state_problem_calculators

Ground state energy calculator using coupled cluster theory.

Classes

CoupledClusterGroundStateProblemCalculator

Class for performing coupled cluster calculations (CCSD or CCSD(T)).

CoupledClusterGroundStateProblemCalculatorOptions

Options to use for coupled cluster ground state calculations.
class CoupledClusterGroundStateProblemCalculator

Bases: object

Class for performing coupled cluster calculations (CCSD or CCSD(T)).

__init__(method: Literal['ccsd', 'ccsd_t'] | None = None, options: CoupledClusterGroundStateProblemCalculatorOptions | None = None) → None

Initialize an instance of CoupledClusterGroundStateProblemCalculator.

Parameters:

  • method (Literal['ccsd', 'ccsd_t'] | None) – The coupled cluster method to use. Supported values are “ccsd” and “ccsd_t”. If None, defaults to “ccsd”.

  • options (CoupledClusterGroundStateProblemCalculatorOptions | None) – Options to use for the calculation. None results in default options.

Return type:

None

calculate(ground_state_problem: RestrictedGroundStateProblem | UnrestrictedGroundStateProblem) → GroundStateProblemCalculatorResult

Calculate the coupled cluster ground state energy.

Depending on the configured method, this performs a CCSD or CCSD(T) calculation. The total energy is:

\(E_\mathrm{total} = E_\mathrm{HF} + E_\mathrm{CC-correlation}\)

For CCSD(T), a perturbative triples correction is added on top of the CCSD correlation energy.

Note that the ground_state_problem may restrict the orbital space, in which case this is not a full calculation but a calculation with a truncated orbital space. In case of a restricted orbital space the inactive energy contribution associated with the active space is included in the ground_state_problem.

The Hamiltonian for the ground state problem is assumed to be real.

Parameters:

ground_state_problem (RestrictedGroundStateProblem | UnrestrictedGroundStateProblem) – The ground state problem to calculate energy for.

Return type:

GroundStateProblemCalculatorResult

class CoupledClusterGroundStateProblemCalculatorOptions

Bases: DataclassPublicAPI

Options to use for coupled cluster ground state calculations.

All fields are immutable (frozen=True) so an instance can be safely reused.

Parameters:

  • canonicality_tolerance – Maximum allowed absolute value of any off-diagonal element of the Fock matrix. Must be set according to SCF convergence thresholds. Default tolerance is set according to default SCF convergence. (default=1e-6)

  • verbose – Verbosity level for the underlying PySCF coupled cluster calculation. Higher values produce more output. (default=0)

__init__(*, canonicality_tolerance: float = 1e-06, verbose: int = 0) → None
Parameters:

  • canonicality_tolerance (float)

  • verbose (int)

Return type:

None

canonicality_tolerance: float = 1e-06
verbose: int = 0