Structure of a simple molecular dynamics FORTRAN program optimized for CRAY vector processing computers

Abstract

A program structure for efficient vectorization of molecular dynamics FORTRAN programs on CRAY vector processing computers is described. Though coded for a very simple pure atomic fluid in a cubic cell with periodic boundary conditions the program can easily be modified to handle more complicated systems. A detailed analysis shows that the present program is faster by 36% for N = 256 particles and faster by more than a factor of 3 for N = 2048 compared with a fully vectorized molecular dynamics program written for the CYBER 205 vector processing machine. In comparison with a link cell MD program also written for a CRAY the program described here runs three times faster for a large particle number N = 6912. This factor increases with decreasing N to 6.3 for N = 1372. The speedup is achieved by 1. i) long vectors in inner loops wherever possible; 2. ii) limiting the number of arithmetic operations in inevitably short loops as much as possible; 3. iii) appropriate library routines; 4. iv) integer index vector neighbour lists.