Cheap and Near Exact CASSCF with Large Active Spaces

Area of Science:

• Quantum chemistry
• Computational physics
• Electronic structure theory

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties.
• Large active spaces in multiconfiguration self-consistent field (MCSCF) calculations pose significant computational challenges.
• Existing methods struggle with efficiency and accuracy for complex systems.

Purpose of the Study:

• To introduce and validate the Heat-bath Configuration Interaction Self-Consistent Field (HCISCF) method.
• To demonstrate the efficiency and accuracy of HCISCF for large active space calculations.
• To investigate the electronic structure of representative molecules using HCISCF.

Main Methods:

• Development and application of the Heat-bath Configuration Interaction (HCI) algorithm as an active space solver.
• Implementation of the HCISCF approach, overcoming non-variationality issues using Lagrangian formulation.
• Calculation of HCI relaxed two-body reduced density matrices for accurate energy determination.

Main Results:

• HCISCF enables efficient multiconfiguration self-consistent field calculations with large active spaces.
• Converged active space orbitals from HCISCF show insensitivity to HCI calculation accuracy.
• Nearly converged CASSCF energies (error < 1 mHa) are achievable with HCISCF.
• HCISCF calculations on Fe-porphyrin with a (44e, 44o) active space are computationally efficient.
• Optimization of active space orbitals using HCISCF accelerates convergence to the Full CI limit.

Conclusions:

• HCISCF is a powerful and efficient method for electronic structure calculations involving large active spaces.
• The method offers a practical route to highly accurate energies for complex molecular systems.
• HCISCF significantly advances the capabilities of computational quantum chemistry.