Singlet-triplet spin-orbit coupling and crossing probability for the single-dimer cluster model of a Si(1Â 0Â 0) surface

► The minimum energy crossing point between singlet and triplet state is located near the higher-energy triplet state. ► The Si-dimer bond length increases as one progresses from the single state to the minimum energy crossing point and finally to the triplet state. ► The moderate spin-orbit coefficient (25 cm−1) at the minimum energy crossing point suggests a small but finite spin-crossing probability between singlet and triplet state at room temperature. ► Small geometry differences at the crossing points located by the different computational methods have insignificant influence on the spin-orbit coefficient. ► At higher temperatures, the population of the triplet Si-dimer is non-negligible, implying that the triplet set could play important roles in chemisorption and decomposition reactions on the Si(1 0 0) surface.