Addressing an instability in unrestricted density functional theory direct dynamics simulations.

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Quantum Chemistry

Background:

• Direct dynamics simulations using Density Functional Theory (DFT) with Unrestricted Hartree Fock (UHF) theory often encounter triplet instability during classical trajectory integration.
• A broken symmetry initial guess for the wave function is a common approach to obtain the unrestricted DFT potential energy surface (PES), but it is frequently insufficient for stable direct dynamics simulations.

Purpose of the Study:

• To develop and describe an algorithm for achieving smooth transitions between open-shell and closed-shell regions of the unrestricted PES.
• To enable stable trajectory calculations in direct dynamics simulations for systems exhibiting such electronic structures.

Main Methods:

• Implementation of an algorithm to manage wave function symmetry during numerical integration.
• Application of Density Functional Theory (DFT) direct dynamics simulations utilizing Unrestricted Hartree Fock (UHF) theory.
• Testing the algorithm on dioxetane and its singlet diradical form.

Main Results:

• The developed algorithm successfully obtains smooth transitions between open-shell and closed-shell regions of the unrestricted PES.
• Stable classical trajectories were achieved in direct dynamics simulations, overcoming the issue of triplet instability.
• The method was validated through simulations of dioxetane and its singlet diradical.

Conclusions:

• The described algorithm effectively resolves triplet instability issues in DFT direct dynamics simulations.
• This approach provides a robust method for calculating stable trajectories on unrestricted potential energy surfaces.
• The findings are significant for accurate computational studies of diradical systems and related chemical reactions.