Potential energy surface determination for non rigid molecules. Two-rotor molecules

Abstract

The potential energy surface determination problem is considered in the case of two rotor molecules from two points of view: 1) The calculation of the energy of each molecular configuration; 2) The application of Group Theory for non-rigid molecules in order to reduce to the strict minimum the configurations to be calculated. From the Group Theory symmetry adapted functional forms are deduced for the potential energy of a set of molecules of different internal symmetry: p-xylene, acetone, dimethylamine, cis-N-methylethylidenimine and trans-1, 2-dimethylcyclopropane. In the same way, minimal expansions, useful for describing the main features of the potential energy surfaces are deduced. From the minimal expansion, the different surfaces are determined num e rically by calculating the energy of a very limited number of nuclear conform a tions. For these last calculations, - barrier height calculations - the CNDO/2 method is found to be the more suitable one, even when compared with the ab initio methods.