B.5 Shell-Quartets and Integral Classes

Given a sorted list of shell-pair data, it is possible to construct all potentially important shell-quartets by pairing of the shell-pairs with one another. Because the shell-pairs have been sorted, it is possible to deal with batches of integrals of the same type or class (e.g., $(ss\vert ss)$ , $(sp\vert sp)$ , $(dd\vert dd)$ , etc.) where an integral class is characterized by both angular momentum ( $L$ ) and degree of contraction ( $K$ ). Such an approach is advantageous for vector processors and for semi-direct integral algorithms where the most expensive (high $K$ or $L$ integral classes can be computed once, stored in memory (or disk) and only less expensive classes rebuilt on each iteration.

While the shell-pairs may have been carefully screened, it is possible for a pair of significant shell-pairs to form a shell-quartet which need not be computed directly. Three cases are:

The quartet is equivalent, by point group symmetry, to another quartet already treated.
The quartet can be ignored on the basis of cheaply computed ERI bounds[Gill et al.(1994)Gill, Johnson, and Pople] on the largest quartet bra-ket.
On the basis of an incremental Fock matrix build, the largest density matrix element which will multiply any of the bra-kets associated with the quartet may be negligibly small.

Note: Significance and negligibility is always based on the level of integral threshold set by the $rem variable THRESH.