Comparison of the Singer method and the constraints method for molecular dynamics with linear molecules on the VECTOR computer Cyber 205

Abstract The vectorization of FORTRAN programmes for the computation of the forces in molecular dynamics (MD) calculations are described. For systems containing linear molecules, two equivalent MD methods can be used: the Singer method and the constraints method. The FORTRAN vector code is presented and discussed for both methods. A comparison of computational times on the CYBER 205 is presented. For the two-centre Lennard-Jones potential, the constraints algorithm becomes increasingly less efficient than the Singer algorithm when executed on the CYBER 205. The reason for this is the difference in the neighbour-list which is made for the centre of each molecule in the Singer method and for each site in the molecule in the constraints method. Both programmes run about a factor of 15 faster on the Cyber 205 than on the conventional computer Cyber 175, for 108 or 256 linear molecules.