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geometry_optimization_optg [2025/09/29 07:42] – remark about singlet-triplet case; see issue github 6119 dollgeometry_optimization_optg [2025/09/29 09:05] (current) – [Conical Intersection optimization (CONICAL)] doll
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-This second example optimizes the singlet-triplet intersection in $LiH_2(+)$ (ground state is Singlet, excited state is Triplet). Though the optimization works for this example, one has to be careful as the intersection between states of different spin is in general a surface when spin-orbit coupling is not included.+This second example optimizes the singlet-triplet intersection in $LiH_2(+)$ (ground state is Singlet, excited state is Triplet). Though the optimization works for this example, one has to be carefulas the intersection between states of different symmetry or spin when spin-orbit coupling is not included leads to an energy difference whose first derivative does not vanish at degeneracy. Most geometry optimisation algorithms assume the opposite.
  
 <code - examples/lih2+_S0T0.inp> <code - examples/lih2+_S0T0.inp>