Quantum Package 2.0

Quantum Package 2.0 implements determinant-driven selected configuration interaction (sCI), the CIPSI algorithm, and multi-reference second-order perturbation theory (PT2) to construct and correct wavefunctions for accurate electronic-structure calculations and extrapolation toward the full configuration interaction (FCI) limit.

Key Features:

• Determinant-driven sCI: Supports arbitrary sets of determinants in the reference space for selected configuration interaction expansions.
• CIPSI with PT2 corrections: Uses the Configuration Interaction using a Perturbative Selection made Iteratively (CIPSI) algorithm to iteratively select determinants and augment variational sCI energies with PT2 corrections.
• Renormalized PT2 correction: Implements a renormalized second-order perturbative correction to improve extrapolation toward the FCI limit.
• Stochastic CIPSI selection: Provides a stochastic version of the CIPSI selection process that operates concurrently with PT2 calculations without additional computational cost.
• Multireference CC and range-separated DFT plugins: Offers external plugins for multireference coupled cluster theory and range-separated density-functional theory.
• Parallel scalability: Supports parallel computations on approximately 2,000 CPU cores, manages tens of millions of determinants in the reference space, and has been run on up to 12,288 cores.
• IRPF90 implementation: Implemented using the IRPF90 code generator.

Scientific Applications:

• Accurate electronic-structure calculations: Generates high-accuracy wavefunctions and energies for correlated molecular systems using sCI combined with PT2 and renormalized corrections.
• Multireference problems: Treats multireference electronic-structure problems via determinant-driven sCI and multi-reference PT2.
• Integration with advanced methods: Enables studies that combine selected CI with multireference coupled cluster theory and range-separated density-functional theory through plugins.
• Extrapolation and benchmarking: Facilitates extrapolation toward the full CI limit and benchmarking of wavefunction methods.

Methodology:

Determinant-driven selected configuration interaction (sCI); CIPSI (Configuration Interaction using a Perturbative Selection made Iteratively) with PT2 corrections; renormalized second-order perturbative correction; stochastic CIPSI selection concurrent with PT2; support for multireference coupled cluster theory and range-separated density-functional theory; implemented using IRPF90.