Si2H2 is an important species that may appear in the chemical vapor deposition of silicon. This paper reports an intrinsic reaction coordinate (IRC), or a minimum energy path (MEP), of the isomerization from silasilene to bridged disilyne obtained by using the local cubic and quadratic approximations. These. new approximations generate a correct IRC for this isomerization, while some conventional methods fail to predict a reasonable IRC because of the very flat potential energy surface. This reaction path bifurcates to two identical IRCs to reach bridged disilyne. The activation energy for this isomerization is predicted to be less than 3 kcalfmol. This energy barrier may disappear at high temperatures. This paper also compares the IRCs on the simple potential energy surfaces of the ammonia inversion and the HCN isomerization generated by using the local cubic and quadratic approximations with those obtained by some conventional methods.