To examine the effects of including quadruple excitations in the electron correlation treatment when computing the binding energy of van der Waals dimers, we have calculated MP2, CCSD, CCSD(T), and CCSD(TQ) interaction energies for several van der Waals complexes ranging from helium dimer to furan dimer. Through comparison of CCSD(TQ) and CCSD(T) binding energies the first direct assessment of the effects of quadruple excitations on π‚‚‚π interactions is presented. The influence of triple excitations is assessed not only in the conventional manner that permeates the literature (comparison of CCSD(T) and MP2 interaction energies) but also via comparison of CCSD(T) and CCSD data. In systems exhibiting significant electron delocalization, it is well-known that triples contributions to binding energy are quite large (on the order of 1 kcal mol -1 ). In these cases, quadruple excitations are nonnegligible and tend to be an order of magnitude smaller than the correction to the binding energy from triple excitations (on the order of 0.1 kcal mol -1 ). The largest quadruples correction observed was +0.2 kcal mol -1 for the furan dimer. † Part of the special issue "Fritz Schaefer Festschrift".
A multicentered integrated QM/QM technique has been developed. By separating high-level calculations in distinct regions of molecules, the multicentered approach supplants a single large high-level calculation with several smaller calculations. Due to the steep polynomial scaling of traditional ab initio quantum chemical methods, this separation significantly enhances the computational efficiency of QM/QM methods. The straightforward implementation of this multicentered approach is illustrated with several large poly-alcohols that form hydrogen bonds with water. The largest alcohol-water complex contains 81 atoms. For properly selected model systems, this multicentered approach introduces essentially no error in the dissociation energies of these complexes relative to conventional QM/QM schemes. This multicentered technique should be easily extended to other, more general integrated methods (QM/MM, ONIOM, etc).
Two new prototype delocalized pi[dot dot dot]pi complexes are introduced: the dimers of cyanogen, (N[triple bond]C-C[triple bond]N)(2), and diacetylene, (HC[triple bond]C-C[triple bond]CH)(2). These dimers have properties similar to larger delocalized pi...pi systems such as benzene dimer but are small enough that they can be probed in far greater detail with high accuracy electronic structure methods. Parallel-slipped and T-shaped structures of both cyanogen dimer and diacetylene dimer have been optimized with 15 different procedures. The effects of basis set size, theoretical method, counterpoise correction, and the rigid monomer approximation on the structure and energetics of each dimer have been examined. MP2 and CCSD(T) optimized geometries for all four dimer structures are reported, as well as estimates of the CCSD(T) complete basis set (CBS) interaction energy for every optimized geometry. The data reported here suggest that future optimizations of delocalized pi[dot dot dot]pi clusters should be carried out with basis sets of triple-zeta quality. Larger basis sets and the expensive counterpoise correction to the molecular geometry are not necessary. The rigid monomer approximation has very little effect on structure and energetics of these dimers and may be used without consequence. Due to a consistent cancellation of errors, optimization with the MP2 method leads to CCSD(T)/CBS interaction energies that are within 0.2 kcal mol(-1) of those for structures optimized with the CCSD(T) method. Future studies that aim to resolve structures separated by a few tenths of a kcal mol(-1) should consider the effects of optimization with the CCSD(T) method.
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