Parametrization of MMX force field for benzocyclobutenes

A detailed geometrical analysis of cyclobuta[a], dicyclobuta[a,c], dicyclob uta[a,d] and tricyclobuta[a,b,c] benzenes by molecular mechanics MMX force field has been performed. Results obtained by optimizing the geometries of mentioned compounds by means of the authentic MMX force field have shown th at it has to be modified. A parametrization of the MMX force field has been performed by correcting the parameters for the sp(3) C-atoms attached to t he benzene ring. Compared to the original MMX force field, the modified ver sion reproduces the geometries of benzocyclobutenes with much greater accur acy and reliability, comparable to those for higher-level ab initio methods . The Mills-Nixon effect (weak but not negligible) is also successfully rep roduced. We hope that such modified force field can be successfully used fo r determining the geometries of various cyclosubstituted benzenes and simil ar compounds.