Vector length

Numerical integration is applied in ADF to evaluate Fock matrix elements and many other quantities that are defined as integrals over basis functions, the charge density, the potential, etc. As a consequence a large part of the CPU time is spent in simple do-loops over the integration points. The total number of points depends on the required precision and on the number of atoms, the geometry and symmetry. All such numerical integration loops are segmented into loops over blocks of points, each block consisting of a certain number of points. This latter defines the most inner do-loop and hence determines vectorization aspects.

Depending on the computer, c.f. the compiler, vector operations may be executed more efficiently using longer vectors. Long vectors increase the demand on Central Memory however because the program may sometimes have to access large numbers of such vectors in combination (for instance all basis functions) so that they must be available in memory simultaneously. The optimum vector length depends therefore on the balance between vectorization efficiency and memory usage. The maximum vector length that you allow the program to use can be set via input.

VECTORLENGTH vectorlength

The default is set at the installation of adf on your platform, see the Installation manual. For organizational reasons the true vector length actually used in the computation may be smaller than the value defined with this key, but will not exceed it (except in a Create run, but in that case performance and memory usage are no hot topics).