Geometry optimizations of open-shell systems with the fragment molecular orbital method

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Molecular Modeling

Background:

• Geometry optimizations are crucial for understanding molecular properties.
• Accurate calculations for open-shell systems, like radicals and excited states, remain computationally challenging.
• Fragment Molecular Orbital (FMO) methods offer a promising approach for large molecular systems.

Purpose of the Study:

• To develop and present restricted open-shell Hartree-Fock (ROHF) gradients within the Fragment Molecular Orbital (FMO) framework.
• To assess the accuracy of the FMO-ROHF gradients for geometry optimizations.
• To investigate the FMO method's capability in reproducing adiabatic excitation energies and evaluate its computational efficiency for large open-shell systems.

Main Methods:

• Implementation of ROHF gradients for the FMO method.
• Testing the accuracy of FMO-ROHF gradients against established methods.
• Calculation and comparison of adiabatic excitation energies using FMO.
• Timing comparisons between FMO and full ab initio calculations.

Main Results:

• The FMO-ROHF gradients were successfully developed and implemented.
• The FMO method demonstrated good accuracy in geometry optimizations for open-shell systems.
• The method showed capability in reproducing adiabatic excitation energies.
• Significant computational efficiency was observed for the FMO method compared to full ab initio calculations for large systems.

Conclusions:

• The FMO method with ROHF gradients provides an accurate and efficient approach for geometry optimizations of large open-shell molecular systems.
• This development expands the applicability of FMO methods to a wider range of chemical species.
• The FMO method offers a viable computational strategy for studying complex open-shell molecules.