CamCASP/Programming/3

CamCASP => Programming => Robust integrals

Outline

We calculate our 4-index 2-electron integrals in the following way (chemical notation):

<math>

 (pq|rs) \approx (\tilde{pq}|\tilde{rs}) = D^A_{pq,k} (k|l) D^B_{rs,l}

</math>

where we have assumed <math>p,q ~ (r,s)</math> belong to molecule A (B). These integrals have errors that are second order in the error in the density. Here's why:

Consider the difference

<math>

 (pq|rs) - (\tilde{pq}|\tilde{rs}) = (pq-\tilde{pq}|rs-\tilde{rs}) + (\tilde{pq}|rs-\tilde{rs}) + (ps-\tilde{pq}|\tilde{rs})

</math>

Since <math>|pq-\tilde{pq})</math> is the error made in fitting transition density <math>|pq)</math>, it appears that the error in this expression is linear in the error in the (transition)-density. But this is not the case, and Dunlap (B.I. Dunlap, Int. J. Quantum Chem. 64, 193 (1997) and Phys. Chem. Chem. Phys. 2, 2113 (2000)) has shown that the terms with linear errors actually vanish. To see this, take one of these terms,

<math>

(\tilde{pq}|rs-\tilde{rs}) 

</math>

together with the expansion <math>|\tilde{pq}) = D_{pq,k} |k)</math> and the density-fitting equations

<math>

 (k|k') D_{pq,k'} = (k|pq).

</math>

So our 2-electron Coulomb integrals are already robust. See Fred Manby's paper in J. Chem. Phys. 119, 4607 (2003) for more details.

Why do we need the robust form?

The main, and probably only reason is that the simple form of these 4-index integrals results in too large an error in the first-order electrostatic energy.