The contraction problem may be described by considering a general contracted ERI of -type functions derived from the STO-3G basis set. Each basis function has degree of contraction = 3. Thus, the ERI may be written

(B.5) |

and requires 81 primitive integrals for the single ERI. The problem escalates dramatically for more highly contracted sets (STO-6G, 6-311G) and has been the motivation for the development of techniques for shell-pair modeling [826], in which a second shell-pair is constructed with fewer primitives that the first, but introduces no extra error relative to the integral threshold sought.

The Pople-Hehre axis-switch method [818] is excellent for high contraction low angular momentum integral classes.