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Constructing Spin-Adiabatic States for the Modeling of Spin-Crossing Reactions I. A Shared-Orbital Implementation

preprint
revised on 19.07.2019 and posted on 22.07.2019 by Yunwen Tao, Zheng Pei, Nicole Bellonzi, Yuezhi Mao, zhu zou, Wanzhen Liang, Zhibo Yang, Yihan Shao
In the modeling of spin-crossing reactions, it has become popular to directly explore the spin-adiabatic surfaces. Specifically, through constructing spin-adiabatic states from a two-state Hamiltonian (with spin-orbit coupling matrix elements) at each geometry, one can readily employ advanced geometry optimization algorithms to acquire a “transition state" structure, where the spin crossing occurs. In this work, we report the implementation of a fully variational spin-adiabatic approach based on Kohn-Sham density functional theory spin states (sharing the same set of molecular orbitals) and the Breit-Pauli one-electron spin-orbit operator. For three model spin-crossing reactions [predissociation of N2O, singlet-triplet conversion in CH2, and CO association to Fe(CO)4], the spin-crossing points were easily obtained. Our results also indicated the Breit-Pauli one-electron spin-orbit coupling can vary significantly along the reaction pathway on the spin-adiabatic energy surface. On the other hand, due to the restriction that low-spin and high-spin states share the same set of molecular orbitals, the acquired spin-adiabatic energy surface shows a cusp (i.e. a first-order discontinuity) at the crossing point, which prevents the use of standard geometry optimization algorithms to pinpoint the crossing point. An extension with this restriction removed is being developed to achieve the smoothness of spin-adiabatic surfaces.

History

Email Address of Submitting Author

yihan.shao@ou.edu

Institution

University of Oklahoma

Country

USA

ORCID For Submitting Author

0000-0001-9337-341X

Declaration of Conflict of Interest

None.

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