Determination of the kinetic energy parameters of non-rigid molecules

The effect of the geometry relaxation on the kinetic energy parameters of a non-rigid molecule Hamiltonian is shown. For this purpose, three species have been selected: hydrogen peroxide, glyoxal and 1,3-butadiene. The torsional Hamiltonians are determined from fully optimized ab initio calculations (MP2/cc-pVTZ). Thus, the dependent internal coordinates convert into functions of the independent variables whose derivatives appear in the kinetic energy calculations, although they are usually neglected. Their effect is significant for small molecules. They are also important for planar molecules where derivatives of the `out-of-planeʹ coordinates cannot be neglected.