A methodol. for the prediction of excitation energies for substituted chromophores from ground state structures was developed. The formalism introduces the concept of structural substituent excitation energy effect for the rational prediction and quantification of the substituent effect in the excitation energy of a chromophore to an excited electronic state. This effect quantifies exclusively the excitation energy variation due to the structural changes of the chromophore induced by the substituent. Therefore, excitation bathochromic and hypsochromic shifts of substituted chromophores can be predicted from known ground and excited potential energy surfaces of a ref. unsubstituted chromophore, together with the ground state min. energy structure of the substituted chromophore. This formalism can be applied if the chem. substitution does not affect the nature of the electronic excitation, where the substituent effect can be understood as a force acting on the chromophore and provoking a structural change on it. The developed formalism provides a useful tool for quant. and qual. detn. of the excitation energy of substituted chromophores and also for the anal. and detn. of the structural changes affecting this energy. The proposed methodol. was applied to the prediction of the excitation energy to the 1st bright state of several S-nitrosothiols using the potential energy surfaces of Me-S-nitrosothiol as a ref. unsubstituted chromophore. [on SciFinder(R)]