# Basis sets optimized for CCSDT(Q) calculations

CCSDT(Q) and CCSDTQ calculations are extraordinarily expensive, both in terms of FLOP count and RAM capacity requirements, eg. CCSDTQ requires a computational effort that scales as ~$$N^{10}$$, where N is the number of (contracted) basis functions. Given that, it is easy to see why running CCSDTQ calculations with fewer basis functions would be advantageous.

The obvious way to reduce N is to use a smaller basis set. While there is some evidence [1,2] that the CCSDT(Q)-CCSDT and especially the CCSDTQ-CCSDT(Q) contributions converge to the basis set limit faster than lower complexity contributions, CCSDT(Q)/cc-pVDZ can still be extremely demanding.

The other alternative would be to create basis sets optimized specifically for the purpose of post-CCSD(T) (CCSDT, CCSDT(Q), ...) calculations, with fewer contracted basis functions per atom. One could really crank up the number of primitive gaussians per basis function, if required, and integral evaluation time would still be utterly negligible compared to the time spent in the CCSDT(Q) code. Something like a hypothetical "ANO-CCQ" basis set might make CSSDT(Q) calculations more feasible.

My question is:

Has anyone ever developed or seen a basis set specifically optimized for post-CCSD(T) calculations?

• My suspicion is that, instead of having a universal basis set that is optimized for post-CCSD(T) calculations, it would be probably better to optimize a system-specific basis set for the specific system at hand, at a lower level (say MP2 or CCSD), and then use that basis set for the post-CCSD(T) calculation. But that's just my personal guess Apr 21 at 17:21
• I am not an expert in coupled-cluster methods but I wanted to point out, that the benefit of using even higher excitations in CC might lie within the error of using a smaller basis Apr 21 at 18:49