Accurate Numerical Computation of Rovibrational G Matrices in Molecules of Arbitrary Size

In this work we present a methodology for the accurate numerical computation of the rovibrational G matrix in any molecule. A C++ program is developed to apply this methodology. Using polymorphism, the program can handle the output of any of the available electronic structure codes. The objective is to compute the kinetic contribution to the rovibrational Hamiltonian from the results of molecular structure scans, performed in heterogeneous and distributed systems such as Internet-based Grids of computers. The numerical derivatives needed to compute the G matrix in curvilinear, internal coordinates are obtained from an adapted Richardson extrapolation. The procedure is optimized to maximize the number of significant digits in the derivatives. Using the program, we compute the vibrational kinetic terms for several simultaneous torsional motions in Glycolaldehyde, Methyl formate and Ethyl methyl ether. The results show the existence of an important coupling among the torsional vibration modes.