We evaluate the performance of the most widely used wavefunction, density functional theory, and semiempirical methods for the description of noncovalent interactions in a set of larger, mostly dispersion-stabilized noncovalent complexes (the L7 data set). The methods tested include MP2, MP3, SCS-MP2, SCS(MI)-MP2, MP2.5, MP2.X, MP2C, DFT-D, DFT-D3 (B3-LYP-D3, B-LYP-D3, TPSS-D3, PW6B95-D3, M06-2X-D3) and M06-2X, and semiempirical methods augmented with dispersion and hydrogen bonding corrections: SCC-DFTB-D, PM6-D, PM6-DH2 and PM6-D3H4. The test complexes are the octadecane dimer, the guanine trimer, the circumcoronene…adenine dimer, the coronene dimer, the guanine-cytosine dimer, the circumcoronene…guanine-cytosine dimer, and an amyloid fragment trimer containing phenylalanine residues. The best performing method is MP2.5 with relative root mean square deviation (rRMSD) of 4 %. It can thus be recommended as an alternative to the CCSD(T)/CBS (alternatively QCISD(T)/CBS) benchmark for molecular systems which exceed current computational capacity. The second best non-DFT method is MP2C with rRMSD of 8 %. A method with the most favorable “accuracy/cost” ratio belongs to the DFT family: BLYP-D3, with an rRMSD of 8 %. Semiempirical methods deliver less accurate results (the rRMSD exceeds 25 %). Nevertheless, their absolute errors are close to some much more expensive methods such as M06-2X, MP2 or SCS(MI)-MP2, and thus their price/performance ratio is excellent.

The performance of the second-order Møller-Plesset perturbation theory MP2.5 and MP2.X methods, tested on the S22, S66, X40, and other benchmark datasets is briefly reviewed. It is found that both methods produce highly accurate binding energies for the complexes contained in these data sets. Both methods also provide reliable potential energy curves for the complexes in the S66 set. Among the routinely used wavefunction methods, the only other technique that consistently produces lower errors, both for stabilization energies and geometry scans, is the spin-component-scaled coupled-clusters method covering iterative single- and double-electron excitations, which is, however, substantially more computationally intensive. The structures originated from full geometrical gradient optimizations at the MP2.5 and MP2.X level of theory were confirmed to be the closest to the CCSD(T)/CBS (coupled clusters covering iterative single- and double-electron excitations and perturbative triple-electron excitations performed at the complete basis set limit) geometries among all the tested methods (e.g. MP3, SCS(MI)-MP2, MP2, M06-2X, and DFT-D method evaluated with the TPSS functional). The MP2.5 geometries for the tested complexes deviate from the references almost negligibly. Inclusion of the scaled third-order correlation energy results in a substantial improvement of the ability to accurately describe noncovalent interactions. The results shown here serve to support the notion that MP2.5 and MP2.X are reasonable alternative methods for benchmark calculations in cases where system size or (lack of) computational resources preclude the use of CCSD(T)/CBS computations. MP2.X allows for the use of smaller basis sets (i.e. 6-31G*) with results that are nearly identical to those of MP2.5 with larger basis sets, which dramatically decreases computation times and makes calculations on much larger systems possible.

The reduction of N,C,N-chelated bismuth chlorides [C6H3-2,6-(CH=NR)2]BiCl2 [where R = tBu (1), 2',6'-Me2C6H3 (2), or 4'-Me2NC6H4 (3)] or N,C-chelated analogues [C6H2-2-(CH=N-2',6'-iPr2C6H3)-4,6-(tBu)2]BiCl2 (4) and [C6H2-2-(CH2NEt2)-4,6-(tBu)2]BiCl2 (5) is reported. Reduction of compounds 1-3 gave monomeric N,C,N-chelated bismuthinidenes [C6H3-2,6-(CH=NR)2]Bi [where R = tBu (6), 2',6'-Me2C6H3 (7) or 4'-Me2NC6H4 (8)]. Similarly, the reduction of 4 led to the isolation of the compound [C6H2-2-(CH=N-2',6'-iPr2C6H3)-4,6-(tBu)2]Bi (9) as an unprecedented two-coordinated bismuthinidene that has been structurally characterized. In contrast, the dibismuthene {[C6H2-2-(CH2NEt2)-4,6-(tBu)2]Bi}2 (10) was obtained by the reduction of 5. Compounds 6-10 were characterized by using (1)H and (13)C NMR spectroscopy and their structures, except for 7, were determined with the help of single-crystal X-ray diffraction analysis. It is clear that the structure of the reduced products (bismuthinidene versus dibismuthene) is ligand-dependent and particularly influenced by the strength of the N→Bi intramolecular interaction(s). Therefore, a theoretical survey describing the bonding situation in the studied compounds and related bismuth(I) systems is included. Importantly, we found that the C3NBi chelating ring in the two-coordinated bismuthinidene 9 exhibits significant aromatic character by delocalization of the bismuth lone pair.