Reducing the Cost of CCSD Basis Set Extrapolation in Ab Initio Computational Thermochemistry

A series of approximations to CCSD contributions in computational
model chemistries is presented in the context of kcal mol–1, kJ mol–1, and 20 cm–1 theoretical
predictions of total atomization energies, benchmarked within the
HEAT+CH4 test suite. A specific set of circumstances where
MP2, without empirical scaling, may be used as an effective intermediate
in the first two of these accuracy ranges was determined. However,
SDQ-MP4, a method long used in pursuit of kcal mol–1 accuracy but relatively unstudied in the subchemical accuracy community,
offers significant improvement over the quality of MP2 as a basis-set
intermediate at significantly reduced cost compared to CCSD. Given
this, we argue for SDQ-MP4 as the de facto CCSD basis-set
intermediate in sub-chemical accuracy calculations when CCSD in a
desired basis set becomes unaffordable. We additionally report on
a “CBS-like” scheme, where MP2 and SDQ-MP4 are used
in conjunction to create a “cheap” three-part approximation
of large CCSD basis set limits. The data for the CCSD approximation
schemes are organized in such a way that model chemistry developers
can locate an analog of their current approach for the CCSD basis
set limit and explore alternative intermediates that either decrease
computational cost or increase computational accuracy. We also show,
for a handful of molecules, that SDQ-MP4 shows promise as an effective
basis-set intermediate for harmonic and fundamental frequency computations,
allowing for zero-point corrections of nearly CCSD­(T)/ANO1 quality
using simple composite methods that only require CCSD­(T)/ANO0.