The use of supercomputers for the variational calculation of ro-vibrationally excited states of floppy molecules

Abstract

The advent of supercomputers has led to great advances in electronic structure calculations and to the ab initio calculation of molecular spectra. Recent theoretical developments have allowed us to develop a two-step variational algorithm for the calculation of rotationally highly excited states of floppy molecules. This algorithm allows highly accurate nuclear motion calculations to be performed on low-lying ro-vibrational states and greatly extends the range of states that can practicably be considered. The algorithm has been adapted to run efficiently on the Cray supercomputers. Analysis of the timings suggest that construction of the secular matrix is highly vectorised and that the special structure of secular matrix can be used to give rapid diagonalisation. The limiting factor on these calculations is the available fast storage, but analysis suggests that this bottleneck could be removed by use of a Solid State Device (SSD). Sample results are given for calculations involving a range of rotational excitation. An adaptation of the algorithm to a loop of parallel processors is also suggested.