Fully variational incremental CASSCF.

Area of Science:

• Computational chemistry
• Electronic structure theory
• Quantum chemistry

Background:

• Complete-active-space self-consistent field (CASSCF) is crucial for first principles simulations but computationally expensive.
• Existing approximations like iCASSCF reduce cost but limit accuracy due to non-variational orbital optimization.
• Accurate nuclear gradients are essential for geometry optimization and reaction pathway studies.

Purpose of the Study:

• To introduce and implement a fully variational iCASSCF method.
• To enable variational optimization of all CASSCF parameters, including orbital gradients.
• To improve the accuracy of nuclear gradients and enable stable geometry optimizations for large systems.

Main Methods:

• Application of the many-body expansion (method of increments) to CASSCF (iCASSCF).
• Development of a formulation allowing full variational optimization of orbital parameters.
• Implementation of the variational iCASSCF method for electronic structure calculations.

Main Results:

• The variational iCASSCF method achieves polynomial scaling while retaining CASSCF accuracy.
• Accurate nuclear gradients are obtained, enabling reliable geometry optimizations.
• The method successfully treats large systems, recovering electronic correlations from full valence spaces.
• Stable geometries and transition states were optimized for challenging test cases.

Conclusions:

• Fully variational iCASSCF offers a powerful and accurate approach for large-scale electronic structure calculations.
• This method overcomes limitations of previous iCASSCF formulations, enhancing its applicability.
• It is a valuable tool for studying complex molecular chemistries and reaction mechanisms.