The Gaussian-3 (G3) composite approach
for thermochemical properties
is revisited in light of the enhanced computational efficiency and
reduced memory costs by applying the resolution-of-the-identity (RI)
approximation for two-electron repulsion integrals (ERIs) to the computationally
demanding component methods in the G3 model: the energy and gradient
computations via the second-order Møller–Plesset perturbation
theory (MP2) and the energy computations using the coupled-cluster
singles–doubles method augmented with noniterative triples
corrections [CCSD(T)]. Efficient implementation of the RI-based methods
is achieved by employing a hybrid distributed/shared memory model
based on MPI and OpenMP. The new variant of the G3 composite approach
based on the RI approximation is termed the RI-G3 scheme, or alternatively
the PDG method. The accuracy of the new RI-G3/PDG scheme is compared
to the “standard” G3 composite approach that employs
the memory-expensive four-center ERIs in the MP2 and CCSD(T) calculations.
Taking the computation of the heats of formation of the closed-shell
molecules in the G3/99 test set as a test case, it is demonstrated
that the RI approximation introduces negligible changes to the mean
absolute errors relative to the standard G3 model (less than 0.1 kcal/mol),
while the standard deviations remain unaltered. The efficiency and
memory requirements for the RI-MP2 and RI-CCSD(T) methods are compared
to the standard MP2 and CCSD(T) approaches, respectively. The hybrid
MPI/OpenMP-based RI-MP2 energy plus gradient computation is found
to attain a 7.5× speedup over the standard MP2 calculations.
For the most demanding CCSD(T) calculations, the application of the
RI approximation is found to nearly halve the memory demand, confer
about a 4–5× speedup for the CCSD iterations, and reduce
the computational time for the compute-intensive triples correction
step by several hours.