Elimination of small terms

This economization scheme avoids the calculation of integrals whose contribution is below a pre-determined threshold, based on density-weighted estimates of the contribution that they may make to the Fock matrix. Consider Coulomb integrals iv,TT g ax,T T ) defined in (161). Fix the exponents on the centres and A, but allow them to mn over all values for A and 

- If the density matrix has changed by AD, each integral makes one direct contribution 

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