We recently formulated a near-linear scaling explicitly correlated coupled cluster singles, doubles, and perturbative triples method applicable to high-spin open-shell states of large molecules based on the pair-natural orbital (PNO) representation of the wavefunction and geminal operators[1]. Through this work, we improve the accuracy of the explicitly correlated energy correction by employing an geminal spanning orbital (GSO) based F12 projector while using the PNOs for the regular wavefunction parameters. The efficiency of the GSOs was demonstrated earlier by some of us[2] for closed-shell systems and we extend this formalism to the open-shell regime within the state of the art DLPNO framework[3]. The new algorithm is near-linear scaling, with no significant additional computational overhead, and reduces the max error and RMSD of a set of 50 challenging open-shell reactions[4] from 3.16 Kcal/mol and 0.99 Kcal/mol to 0.71 Kcal/mol and 0.23 Kcal/mol respectively for the aug-cc-pVT(+d)Z basis compared to the corresponding canonical explicitly correlated coupled cluster results. Our method can also calculate highly accurate coupled cluster energies of large open-shell molecules with more than 500 atoms and 5000 basis functions on a multi-core computer in less than 3 days.

Improving explicitly correlated geminals for open-shell coupled-cluster pair-natural orbital methods

ACS Fall 2021

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