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Constructing spin‐adiabatic states for the modeling of spin‐crossing reactions. I. A shared‐orbital implementation
Author(s) -
Tao Yunwen,
Pei Zheng,
Bellonzi Nicole,
Mao Yuezhi,
Zou Zhu,
Liang Wanzhen,
Yang Zhibo,
Shao Yihan
Publication year - 2020
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.26123
Subject(s) - atomic orbital , physics , adiabatic process , spin states , spin (aerodynamics) , pauli exclusion principle , hamiltonian (control theory) , molecular orbital , quantum mechanics , atomic physics , electron , molecule , mathematics , mathematical optimization , thermodynamics
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 N 2 O, singlet‐triplet conversion in CH 2 , 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.