Boosting Accuracy with Floating Orbitals: A New Approach for Large Molecular Complexes

Recent progress in local electron correlation methods has made it easier to perform CCSD(T) calculations on large molecules, but achieving the complete basis set (CBS) limit remains challenging due to basis set superposition errors. This is where non-atom-centered or floating orbitals (FOs) come in. FOs are not widely used because they're tough to define, but they can help. Previous methods placed a single FO center, which isn't ideal for larger complexes. A recent alternative uses a grid of FO centers, but it's only for specific atom types.

Building on these ideas, a new approach uses a layer of FO centers with 4-9 FOs per center for each monomer. This creates a double layer of FOs between interacting subsystems. By extending a double-ζ atomic orbital (AO) basis with this double layer of FOs, the quality of conventional augmented double-ζ or triple-ζ AO bases can be matched or even surpassed with fewer orbitals. This results in minimal basis set errors, even for medium-sized dimers. This good performance also applies to larger molecules, up to 72 atoms. Efficient local natural orbital (LNO) CCSD(T) computations with just double-ζ AO and 4 FOs per center match the LNO-CCSD(T)/CBS reference within 0. 1 kcal/mol. This means FO methods can accurately model large molecular complexes without atom type limitations, speeding up efficient correlation calculations like LNO-CCSD(T).

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