Basis Set Dependence of Interaction Energies Computed Using Composite Post-MP2 Methods

Benchmarking Swaths of Intermolecular Interaction Components with Symmetry-Adapted Perturbation Theory

A benchmark database for interaction energy components of various noncovalent interactions (NCIs) along their dissociation curve is one of the essential needs in theoretical chemistry, especially for the development of force fields and machine-learning methods. We utilize DFT-SAPT or SAPT(DFT) as one of the most accurate methods to generate an extensive stock of the energy components, including dispersion energies extrapolated to the complete basis set limit (CBS). Precise analyses of the created data, and benchmarking the total interaction energies against the best available CCSD(T)/CBS values, reveal different aspects of the methodology and the nature of NCIs. For example, error cancellation effects between the S2 approximation and nonexact xc-potentials occur, and large charge transfer energies in some systems, including heavy atoms, can explain the lower accuracy of DFT-SAPT. This method is perfect for neutral complexes containing light nonmetals, while other systems with heavier atoms should be treated carefully. In the last part, a representative data set for all NCIs is extracted from the original data.

Energy, orbital and structural stacking landscape of a purine homodimer system

The multidimensional study, combining the extensive calculations of potential energy surfaces for the parallel-displaced configurations and methods such as energy decomposition and natural bond orbital analysis, has been carried out. The resulted data give an energy, orbital and structural landscapes of this biologically essential system. The balance of the two energy sources, electrostatic and dispersion, is clearly visible. The obtained results, taken as a whole, provide an insight into the hierarchy of intermolecular interactions in the purine system, together with their sources.

Modeling Polymorphic Molecular Crystals with Electronic Structure Theory

Interest in molecular crystals has grown thanks to their relevance to pharmaceuticals, organic semiconductor materials, foods, and many other applications. Electronic structure methods have become an increasingly important tool for modeling molecular crystals and polymorphism. This article reviews electronic structure techniques used to model molecular crystals, including periodic density functional theory, periodic second-order Møller-Plesset perturbation theory, fragment-based electronic structure methods, and diffusion Monte Carlo. It also discusses the use of these models for predicting a variety of crystal properties that are relevant to the study of polymorphism, including lattice energies, structures, crystal structure prediction, polymorphism, phase diagrams, vibrational spectroscopies, and nuclear magnetic resonance spectroscopy. Finally, tools for analyzing crystal structures and intermolecular interactions are briefly discussed.

The S66x8 benchmark for noncovalent interactions revisited: explicitly correlated ab initio methods and density functional theory

The S66x8 dataset for noncovalent interactions of biochemical relevance has been re-examined by means of CCSD(F12*)(T), DFT, and SAPT methods.

At what chain length do unbranched alkanes prefer folded conformations?

Short unbranched alkanes are known to prefer linear conformations, whereas long unbranched alkanes are folded. It is not known with certainty at what chain length the linear conformation is no longer the global minimum. To clarify this point, we use ab initio and density functional methods to compute the relative energies of the linear and hairpin alkane conformers for increasing chain lengths. Extensive electronic structure calculations are performed to obtain optimized geometries, harmonic frequencies, and accurate single point energies for the selected alkane conformers from octane through octadecane. Benchmark CCSD(T)/cc-pVTZ single point calculations are performed for chains through tetradecane, whereas approximate methods are required for the longer chains up to octadecane. Using frozen natural orbitals to unambiguously truncate the virtual orbital space, we are able to compute composite CCSD FNO(T) single point energies for all the chain lengths. This approximate composite method has significant computational savings compared to full CCSD(T) while retaining ∼0.15 kcal/mol accuracy compared to the benchmark results. More approximate dual-basis resolution-of-the-identity double-hybrid DFT calculations are also performed and shown to have reasonable 0.2-0.4 kcal/mol errors compared with our benchmark values. After including contributions from temperature dependent internal energy shifts, we find the preference for folded conformations to lie between hexadecane and octadecane, in excellent agreement with recent experiments [ Lüttschwager , N. O. ; Wassermann , T. N. ; Mata , R. A. ; Suhm , M. A. Angew. Chem. Int. Ed. 2013 , 52 , 463 ].

Exploring the C-X…π halogen bonding motif: an infrared and Raman study of the complexes of CF₃X (X = Cl, Br and I) with the aromatic model compounds benzene and toluene

The formation of halogen bonded complexes formed between the trifluorohalomethanes CF₃Cl, CF₃Br and CF₃I and the Lewis bases benzene and toluene at temperatures below 150K was investigated using FTIR and Raman spectroscopy. Experiments using liquid krypton as solvent show that for both CF₃Br and CF₃I substantial fractions of the monomers can be involved in 1:1 complexes. In addition, weak absorptions illustrating the formation of 2:1 complexes between CF₃I and benzene are observed. Using spectra recorded at temperatures between 120 and 140 K, observed information on the relative stability was obtained for all complexes by determining the complexation enthalpies in solution. The resulting values for CF₃Br.benzene, CF₃I.benzene and (CF₃I)₂.benzene are -6.5(3), -7.6(2) and -14.5(9) kJ mol⁻¹. The values for CF₃Br.toluene and CF₃I.toluene are -6.2(5) and -7.4(5) kJ mol⁻¹. The experimental complexation enthalpies are compared with theoretical data obtained by combining results from MP2/aug-cc-pVDZ(-PP) and MP2/aug-cc-pVTZ(-PP) ab initio calculations, from statistical thermodynamical calculations and from Monte Carlo Free Energy Perturbation simulations. The data are also compared with results derived for other C-X···π halogen bonded complexes involving unsaturated Lewis bases such as ethene and ethyne.