Constructing spin-adiabatic states for the modeling of spin-crossing reactions. I. A shared-orbital implementation |
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| Authors: | Yunwen Tao Zheng Pei Nicole Bellonzi Yuezhi Mao Zhu Zou Wanzhen Liang Zhibo Yang Yihan Shao |
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| Affiliation: | 1. Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma;2. State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, China;3. Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania;4. Department of Chemistry, Stanford University, Stanford, California |
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| Abstract: | 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 addition to Fe(CO)4), the spin-crossing points were 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 (ie, 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. |
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| Keywords: | chemical reactions spin-adiabatic surface spin-crossing reaction |
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