as the NAC vector between two excited states of the given spin multiplicity σ. It is noted that the second derivatives of Ψ r or Λ r with respet to the nuclei coordinates have been neglected. If the first term, the direct SOC, vanishes, this intersystem crossing is spin forbidden. The second term denotes to the nuclear derivatives of a given SOCME between initial and final states with different spin. And if it gets involved in the rate calculations, we refer its contribution as the HTVC effect in this article.