A geometrical model for estimating the effects of external fields upon a reaction path is proposed. The reaction path is defined as the intrinsic reaction coordinate (IRC) which is treated as a “string”. The IRC frame is introduced in order to uniquely determine the attitude of the string with respect to the reaction medium. The string is thrown in the external fields of the reaction medium, and slides or rotates, and is deformed. The cell structure attached to the string is also deformed. A perturbational approach to the reaction rate formula is presented.