Lőrincz Balázs D, Nagy Péter R
Department of Physical Chemistry and Materials Science, Faculty of Chemical Technology and Biotechnology, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary.
HUN-REN-BME Quantum Chemistry Research Group, Müegyetem rkp. 3., H-1111 Budapest, Hungary.
J Phys Chem A. 2024 Nov 28;128(47):10282-10298. doi: 10.1021/acs.jpca.4c04689. Epub 2024 Nov 18.
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).
局部电子相关方法的最新进展使得对含有一百个或更多原子的分子进行相对常规的CCSD(T)（即具有单、双和微扰三激发的耦合簇）计算成为可能。在此，由于广泛的基组叠加误差，接近其完全基组（CBS）极限变得更具挑战性，这通常需要使用带有弥散函数的大原子轨道（AO）基组。在此，我们研究一种以非原子中心或浮动轨道（FO）形式存在的潜在补救方法。由于定义其位置、数量、指数等实际复杂性，即使对于小分子，FO也很少被使用。因此，最常用的FO方法只是在非共价二聚体中间放置一个带有大量FO的单个FO中心；然而，对于更大的复合物，单个FO中心很快就会变得不够用。最近的一种替代方法是在单体周围使用FO中心网格，每个中心有一个单s函数，目前仅适用于H、C、N和O原子。在此，我们基于上述优点并通过为每个单体使用一层FO中心和每个中心4 - 9个FO来减轻先前FO方法的一些缺点。因此，在相互作用的子系统之间放置了双层FO。当用这双层FO扩展双ζ AO基组时，可以用更少的轨道达到或超过传统增强双ζ或传统三ζ AO基组的质量，对于中等大小的二聚体，基组误差可达十分之几kcal/mol。这种良好的性能扩展到更大的分子（此处展示了多达72个原子的分子），因为仅使用双ζ AO和每个中心4个FO的基组进行高效的局部自然轨道（LNO）CCSD(T)计算与我们的LNO - CCSD(T)/CBS参考值在约0.1 kcal/mol内匹配。这些进展通过进一步加速高效的相关计算（如LNO - CCSD(T)），将FO方法引入到对大分子复合物的精确建模中，且不受原子类型的限制。
