Advancing Non-Atom-Centered Basis Methods for More Accurate Interaction Energies: Benchmarks and Large-Scale Applications

Recent advances in local electron correlation approaches
have enabled
the relatively routine access to CCSD(T) [that is, coupled cluster
(CC) with single, double, and perturbative triple excitations] computations
for molecules of a hundred or more atoms. Here, approaching their
complete basis set (CBS) limit becomes more challenging due to extensive
basis set superposition errors, often necessitating the use of large
atomic orbital (AO) basis sets with diffuse functions. Here, we study
a potential remedy in the form of non-atom-centered or floating orbitals
(FOs). FOs are still rarely employed even for small molecules due
to the practical complication of defining their position, number,
exponents, etc. The most frequently used FO method thus simply places
a single FO center with a large number of FOs toward the middle of
noncovalent dimers; however, a single FO center for larger complexes
can soon become insufficient. A recent alternative uses a grid of
FO centers around the monomers with a single s function per center,
which is currently applicable only for H, C, N, and O atoms. Here,
we build on the above advantages and mitigate some drawbacks of previous
FO approaches by using a layer of FO centers and 4–9 FOs/center
for each monomer. Thus, a double layer of FOs is placed between the
interacting subsystems. When extending the double-ζ AO basis
with this double layer of FOs, the quality of conventional augmented
double-ζ or conventional triple-ζ AO bases can be reached
or surpassed with less orbitals, leading to few tenths of a kcal/mol
basis set errors for medium-sized dimers. This good performance extends
to larger molecules (shown here up to 72 atoms), as efficient local
natural orbital (LNO) CCSD(T) computations with only double-ζ
AO and 4 FOs/center FO bases match our LNO–CCSD(T)/CBS reference
within ca. 0.1 kcal/mol. These developments introduce FO methods to
the accurate modeling of large molecular complexes without limitations
to atom types by further accelerating efficient correlation calculations,
like LNO–CCSD(T).