Journal Article bifurcation density functional theory potential energy surface Anharmonic downward distortion Beckmann rearrangement Tully's fewest switches algorithm Zhu-Nakamura global switching algorithm ab initio molecular dynamics hydride intersystem crossing long-range correction natural reaction orbital orbital density analysis organic bond formation reactions quality test of functionals reaction path reactive orbital energy theory spin-orbit coupling substituent effect surface hopping the electronic theory of organic chemistry total density analysis transition state