Computations of Noncovalent π Interactions

High-Order Quantum-Mechanical Analysis of Hydrogen Bonding in Hachimoji and Natural DNA Base Pairs

High-order quantum chemistry is applied to hydrogen-bonded natural DNA nucleobase pairs [adenine:thymine (A:T) and guanine:cytosine (G:C)] and non-natural Hachimoji nucleobase pairs [isoguanine:1-methylcytosine (B:S) and 2-aminoimidazo[1,2a][1,3,5]triazin-4(1H)-one:6-amino-5-nitropyridin-2-one (P:Z)] to see how the intermolecular interaction energies and their energetic components (electrostatics, exchange-repulsion, induction/polarization, and London dispersion interactions) vary among the base pairs. We examined the Hoogsteen (HG) geometries in addition to the traditional Watson-Crick (WC) geometries. Coupled-cluster theory through perturbative triples [CCSD(T)] extrapolated to the complete basis set (CBS) limit and high-order symmetry-adapted perturbation theory (SAPT) at the SAPT2+(3)(CCD)δMP2/aug-cc-pVTZ level are used to estimate highly accurate noncovalent interaction energies. Electrostatic interactions are the most attractive component of the interaction energies, but the sum of induction/polarization and London dispersion is nearly as large, for all base pairs and geometries considered. Interestingly, the non-natural Hachimoji base pairs interact more strongly than the corresponding natural base pairs, by -21.8 (B:S) and -0.3 (P:Z) kcal mol^-1 in the WC geometries, according to CCSD(T)/CBS. This is consistent with the H-bond distances being generally shorter in the non-natural base pairs. The natural base pairs are energetically more stabilized in their Hoogsteen geometries than in their WC geometries. The Hoogsteen geometry makes the A:T base pair slightly more stable, by -0.8 kcal mol^-1, and it greatly stabilizes the G:C⁺ base pair, by -15.3 kcal mol^-1. The G:C⁺ stabilization is mainly due to the fact that C has typically added a proton when found in Hoogsteen geometries. By contrast, Hoogsteen geometries are substantially less favorable than WC geometries for non-natural Hachimoji base pairs, by 17.3 (B:S) and 13.8 (P:Z) kcal mol^-1.

Quantum mechanical study of interactions between sunscreen ingredients and nucleotide bases

Interactions between the popular sunscreen ingredients oxybenzone and homosalate and DNA bases have been studied using density functional theory and ab initio methods. Low-energy structures for each sunscreen ingredient interacting with each nucleotide base in either a pi-stacked or hydrogen-bonded fashion were found. The binding energies are comparable to those for the Watson-Crick-Franklin Ade-Thy and Cyt-Gua pairs. Pi-stacked and hydrogen-bonded structures are comparable in energy, with hydrogen-bonded structures having a more negative counterpoise-corrected binding energy, while the final pi-stacked structures are lower in energy. This is due to a geometrical rearrangement required to form the hydrogen bonds that raise the total energy of the complex. It was also found that when using the M06-2X density functional, the STO-3G basis set favors hydrogen bonding, but 6-31G(d) and 6-31 + G(s) basis sets predict similar binding geometries.

Elastic properties of the molecular crystals of hydrocarbons from first principles calculations

I studied the elastic properties of oligoacenes and polycyclic aromatic hydrocarbons crystals within the framework of density functional theory with van der Waals interactions. The full sets of elastic constants were computed. The computed parameters have good agreement with experimental data. The study of four forms (α, β, γ and dimeric) demonstrate that the stacking of molecules leads to different character of interaction between molecules. Crystals with dimeric form have lower anisotropy. If symmetry of crystals is the same, the increase in number of aromatic rings results in anisotropy increase.

A General Route to Include Pauli Repulsion and Quantum Dispersion Effects in QM/MM Approaches

A methodology to account for nonelectrostatic interactions in Quantum Mechanical (QM)/Molecular Mechanics (MM) approaches is developed. Formulations for Pauli repulsion and dispersion energy, explicitly depending on the QM density, are derived. Such expressions are based on the definition of an auxiliary density on the MM portion and the Tkatchenko-Scheffler (TS) approach, respectively. The developed method is general enough to be applied to any QM/MM method and partition, provided an accurate tuning of a small number of parameters is obtained. The coupling of the method with both nonpolarizable and fully polarizable QM/fluctuating charge (FQ) approaches is reported and applied. A suitable parametrization for the aqueous solution, so that its most representative features are well reproduced, is outlined. Then, the obtained parametrization and method are applied to calculate the nonelectrostatic (repulsion and dispersion) interaction energy of nicotine in aqueous solution.

Formation of Potential Interstellar Noble Gas Molecules in Gas and Adsorbed Phases

The discovery of naturally occurring ArH⁺ in various regions of the interstellar medium has shown the need for more understanding of the reactions that lead to covalently bonded noble gas molecules. The test comes with trying to predict the formation of other small noble gas molecules. Many molecules have been observed in various interstellar environments, which possess the possibility of bonding with noble gases. This work explores how both argon and neon can form bonds to ligands made of these species through quantum chemical computations. Argon and neon are chosen as they are among the most abundant atoms in the universe but are more polarizable than the more common but smaller helium atom. Reactions leading to noble gas molecules are modeled in the gas phase as well as through the adsorbed phase by catalysis with a polycyclic aromatic hydrocarbon (PAH) surface. The adsorption energy of the neutral noble gas atoms to the surface increases as the size of the PAH also increases but this is still less than 10 kcal/mol. It is proposed and supported herein that an incoming molecule can bond with the noble gas atom adsorbed onto the PAH, form a stable structure, and allow the PAH to function as the leaving group. This work shows that the noble gas molecules ArCCH⁺, ArOH⁺, ArNH⁺, and NeCCH⁺ are not only stable minima on their respective potential energy surfaces but also can be formed in either the gas phase or through PAH adsorption with known or hypothesized interstellar molecules. Most notably, NeCCH⁺ does not appear to form in the gas phase but could be catalyzed on PAH surfaces. Hence, the interstellar detection of such molecules could also serve as a probe for the observation of interstellar PAHs.

Assessment of DFT Functionals for QTAIM Topological Analysis of Halogen Bonds with Benzene

Halogen bonding, a noncovalent interaction between a halogen atom and a nucleophilic site, is receiving a growing attention in the chemical community stimulating a large number of theoretical investigations. The density functional theory (DFT) approach revealed to be one of the most suitable methods owing to its accuracy and low computational cost. We report here a detailed analysis of the performance of an extensive set of DFT functionals in reproducing accurate binding energies and topological properties for the halogen-bonding interaction of either NCX or PhX molecules (X = F, Cl, Br, I) with the aromatic system of benzene in the T-shaped configuration. It was found that the better performance for both sets of properties is provided by a small subset of functionals able to take into account, implicitly or explicitly (by inclusion of an additive pairwise potential), the dispersion contribution, that is, ωB97X, M06-2X, M11, mPW2PLYP-D, and B2PLYP-D3.

Excited State Trends in Bidirectionally Expanded Closed-Shell PAH and PANH Anions

Some anions are known to exhibit excited states independent of external forces such as dipole moments and induced polarizabilities. Such states exist simply as a result of the stabilization of valence accepting orbitals whereby the binding energy of the extra electron is greater than the valence excitation energy. Closed-shell anions are interesting candidates for such transitions since their ground-state, spin-paired nature makes the anions more stable from the beginning. Consequently, this work shows the point beyond which deprotonated, closed-shell polycyclic aromatic hydrocarbons (PAHs) and those PAHs containing nitrogen heteroatoms (PANHs) will exhibit valence excited states. This behavior has already been demonstrated in some PANHs and for anistropically extended PAHs. This work establishes a general trend for PAHs/PANHs of arbitrary size and directional extension, whether in one dimension or two. Once seven six-membered rings make up a PAH/PANH, valence excited states are present. For most classes of PAHs/PANHs, this number is closer to four. Even though most of these excited states are weak absorbers, the sheer number of PAHs present in various astronomical environments should make them significant contributors to astronomical spectra.

Spectroscopic and Theoretical Study of Spin-Dependent Electron Transfer in an Iron(III) Superoxo Complex

It was shown previously (J. Am. Chem. Soc. 2014, 136, 10846) that bubbling of O2 into a solution of Fe(II)(BDPP) (H2BDPP = 2,6-bis[[(S)-2-(diphenylhydroxymethyl)-1-pyrrolidinyl]methyl]pyridine) in tetrahydrofuran at -80 °C generates a high-spin (SFe = (5)/2) iron(III) superoxo adduct, 1. Mössbauer studies revealed that 1 is an exchange-coupled system, [Formula: see text], where SR = (1)/2 is the spin of the superoxo radical, of which the spectra were not well enough resolved to determine whether the coupling was ferromagnetic (S = 3 ground state) or antiferromagnetic (S = 2). The glass-forming 2-methyltetrahydrofuran solvent yields highly resolved Mössbauer spectra from which the following data have been extracted: (i) the ground state of 1 has S = 3 (J < 0); (ii) |J| > 15 cm(-1); (iii) the zero-field-splitting parameters are D = -1.1 cm(-1) and E/D = 0.02; (iv) the major component of the electric-field-gradient tensor is tilted ≈7° relative to the easy axis of magnetization determined by the MS = ±3 and ±2 doublets. The excited-state MS = ±2 doublet yields a narrow parallel-mode electron paramagnetic resonance signal at g = 8.03, which was used to probe the magnetic hyperfine splitting of (17)O-enriched O2. A theoretical model that considers spin-dependent electron transfer for the cases where the doubly occupied π* orbital of the superoxo ligand is either "in" or "out" of the plane defined by the bent Fe-OO moiety correctly predicts that 1 has an S = 3 ground state, in contrast to the density functional theory calculations for 1, which give a ground state with both the wrong spin and orbital configuration. This failure has been traced to a basis set superposition error in the interactions between the superoxo moiety and the adjacent five-membered rings of the BDPP ligand and signals a fundamental problem in the quantum chemistry of O2 activation.

Stabilizing and Modulating Color by Copigmentation: Insights from Theory and Experiment

Natural anthocyanin pigments/dyes and phenolic copigments/co-dyes form noncovalent complexes, which stabilize and modulate (in particular blue, violet, and red) colors in flowers, berries, and food products derived from them (including wines, jams, purees, and syrups). This noncovalent association and their electronic and optical implications constitute the copigmentation phenomenon. Over the past decade, experimental and theoretical studies have enabled a molecular understanding of copigmentation. This review revisits this phenomenon to provide a comprehensive description of the nature of binding (the dispersion and electrostatic components of π-π stacking, the hydrophobic effect, and possible hydrogen-bonding between pigment and copigment) and of spectral modifications occurring in copigmentation complexes, in which charge transfer plays an important role. Particular attention is paid to applications of copigmentation in food chemistry.

An efficient algorithm for multipole energies and derivatives based on spherical harmonics and extensions to particle mesh Ewald

Next-generation molecular force fields deliver accurate descriptions of non-covalent interactions by employing more elaborate functional forms than their predecessors. Much work has been dedicated to improving the description of the electrostatic potential (ESP) generated by these force fields. A common approach to improving the ESP is by augmenting the point charges on each center with higher-order multipole moments. The resulting anisotropy greatly improves the directionality of the non-covalent bonding, with a concomitant increase in computational cost. In this work, we develop an efficient strategy for enumerating multipole interactions, by casting an efficient spherical harmonic based approach within a particle mesh Ewald (PME) framework. Although the derivation involves lengthy algebra, the final expressions are relatively compact, yielding an approach that can efficiently handle both finite and periodic systems without imposing any approximations beyond PME. Forces and torques are readily obtained, making our method well suited to modern molecular dynamics simulations.

Understanding substituent effects in noncovalent interactions involving aromatic rings

Noncovalent interactions involving aromatic rings such as π-stacking, cation/π, and anion/π interactions are central to many areas of modern chemistry. Decades of experimental studies have provided key insights into the impact of substituents on these interactions, leading to the development of simple intuitive models. However, gas-phase computational studies have raised some doubts about the physical underpinnings of these widespread models. In this Account we review our recent efforts to unravel the origin of substituent effects in π-stacking and ion/π interactions through computational studies of model noncovalent dimers. First, however, we dispel the notion that so-called aromatic interactions depend on the aromaticity of the interacting rings by studying model π-stacked dimers in which the aromaticity of one of the monomers can be "switched off". Somewhat surprisingly, the results show that not only is aromaticity unnecessary for π-stacking interactions, but it actually hinders these interactions to some extent. Consequently, when thinking about π-stacking interactions, researchers should consider broader classes of planar molecules, not just aromatic systems. Conventional models maintain that substituent effects in π-stacking interactions result from changes in the aryl π-system. This view suggests that π-stacking interactions are maximized when one ring is substituted with electron-withdrawing groups and the other with electron donors. In contrast to these prevailing models, we have shown that substituent effects in π-stacking interactions can be described in terms of direct, local interactions between the substituents and the nearby vertex of the other arene. As a result, in polysubstituted π-stacked dimers the substituents operate independently unless they are in each other's local environment. This means that in π-stacked dimers in which one arene is substituted with electron donors and the other with electron acceptors the interactions will be enhanced only to the extent provided by each substituent on its own, unless the substituents on opposing rings are in close proximity. Overall, this local, direct interaction model predicts that substituent effects in π-stacking interactions will be additive and transferable and will also depend on the relative position of substituents on opposing rings. For cation/π and anion/π interactions, similar π-resonance-based models pervade the literature. Again, computational results indicate that substituent effects in model ion/π complexes can be described primarily in terms of direct interactions between the ion and the substituent. Changes in the aryl π-system do not significantly affect these interactions. We also present a simple electrostatic model that further demonstrates this effect and suggests that the dominant interaction for simple substituents is the interaction of the charged ion with the local dipole associated with the substituents. Finally, we discuss substituent effects in electrostatic potentials (ESPs), which are widely used in discussions of noncovalent interactions. In the past, widespread misconceptions have confused the relationship between changes in ESPs and local changes in the electron density. We have shown that computed ESP plots of diverse substituted arenes can be reproduced without altering the aryl π-density. This is because substituent-induced changes in the ESP above the center of aryl rings result primarily from through-space effects of substituents rather than through changes in the distribution of the π-electron density.

Symmetric and asymmetric triple excitation corrections for the orbital-optimized coupled-cluster doubles method: improving upon CCSD(T) and CCSD(T)(Λ): preliminary application

Symmetric and asymmetric triple excitation corrections for the orbital-optimized coupled-cluster doubles (OO-CCD or simply "OD" for short) method are investigated. The conventional symmetric and asymmetric perturbative triples corrections [(T) and (T)(Λ)] are implemented, the latter one for the first time. Additionally, two new triples corrections, denoted as OD(Λ) and OD(Λ)(T), are introduced. We applied the new methods to potential energy surfaces of the BH, HF, C(2), N(2), and CH(4) molecules, and compare the errors in total energies, with respect to full configuration interaction, with those from the standard coupled-cluster singles and doubles (CCSD), with perturbative triples [CCSD(T)], and asymmetric triples correction (CCSD(T)(Λ)) methods. The CCSD(T) method fails badly at stretched geometries, the corresponding nonparallelity error is 7-281 kcal mol(-1), although it gives reliable results near equilibrium geometries. The new symmetric triples correction, CCSD(Λ), noticeably improves upon CCSD(T) (by 4-14 kcal mol(-1)) for BH, HF, and CH(4); however, its performance is worse than CCSD(T) (by 1.6-4.2 kcal mol(-1)) for C(2) and N(2). The asymmetric triples corrections, CCSD(T)(Λ) and CCSD(Λ)(T), perform remarkably better than CCSD(T) (by 5-18 kcal mol(-1)) for the BH, HF, and CH(4) molecules, while for C(2) and N(2) their results are similar to those of CCSD(T). Although the performance of CCSD and OD is similar, the situation is significantly different in the case of triples corrections, especially at stretched geometries. The OD(T) method improves upon CCSD(T) by 1-279 kcal mol(-1). The new symmetric triples correction, OD(Λ), enhances the OD(T) results (by 0.01-2.0 kcal mol(-1)) for BH, HF, and CH(4); however, its performance is worse than OD(T) (by 1.9-2.3 kcal mol(-1)) for C(2) and N(2). The asymmetric triples corrections, OD(T)(Λ) and OD(Λ)(T), perform better than OD(T) (by 2.0-6.2 kcal mol(-1)). The latter method is slightly better for the BH, HF, and CH(4) molecules. However, for C(2) and N(2) the new results are similar to those of OD(T). For the BH, HF, and CH(4) molecules, OD(Λ)(T) provides the best potential energy curves among the considered methods, while for C(2) and N(2) the OD(T) method prevails. Hence, for single-bond breaking the OD(Λ)(T) method appears to be superior, whereas for multiple-bond breaking the OD(T) method is better.

Supramolecular binding thermodynamics by dispersion-corrected density functional theory

The equilibrium association free enthalpies ΔG(a) for typical supramolecular complexes in solution are calculated by ab initio quantum chemical methods. Ten neutral and three positively charged complexes with experimental ΔG(a) values in the range 0 to -21 kcal mol(-1) (on average -6 kcal mol(-1)) are investigated. The theoretical approach employs a (nondynamic) single-structure model, but computes the various energy terms accurately without any special empirical adjustments. Dispersion corrected density functional theory (DFT-D3) with extended basis sets (triple-ζ and quadruple-ζ quality) is used to determine structures and gas-phase interaction energies (ΔE), the COSMO-RS continuum solvation model (based on DFT data) provides solvation free enthalpies and the remaining ro-vibrational enthalpic/entropic contributions are obtained from harmonic frequency calculations. Low-lying vibrational modes are treated by a free-rotor approximation. The accurate account of London dispersion interactions is mandatory with contributions in the range -5 to -60 kcal mol(-1) (up to 200% of ΔE). Inclusion of three-body dispersion effects improves the results considerably. A semilocal (TPSS) and a hybrid density functional (PW6B95) have been tested. Although the ΔG(a) values result as a sum of individually large terms with opposite sign (ΔE vs. solvation and entropy change), the approach provides unprecedented accuracy for ΔG(a) values with errors of only 2 kcal mol(-1) on average. Relative affinities for different guests inside the same host are always obtained correctly. The procedure is suggested as a predictive tool in supramolecular chemistry and can be applied routinely to semirigid systems with 300-400 atoms. The various contributions to binding and enthalpy-entropy compensations are discussed.

The assessment and application of an approach to noncovalent interactions: the energy decomposition analysis (EDA) in combination with DFT of revised dispersion correction (DFT-D3) with Slater-type orbital (STO) basis set

An assessment study is presented about energy decomposition analysis (EDA) in combination with DFT including revised dispersion correction (DFT-D3) with Slater-type orbital (STO) basis set. There has been little knowledge about the performance of the EDA + DFT-D3 concerning STOs. In this assessment such an approach was applied to calculate noncovalent interaction energies and their corresponding components. Complexes in S22 set were used to evaluate the performance of EDA in conjunction with four representative types of GGA-functionals of DFT-D3 (BP86-D3, BLYP-D3, PBE-D3 and SSB-D3) with three STO basis sets ranging in complexity from DZP, TZ2P to QZ4P. The results showed that the approach of EDA + BLYP-D3/TZ2P has a better performance not only in terms of calculating noncovalent interaction energy quantitatively but also in analyzing corresponding energy components qualitatively. This approach (EDA + BLYP-D3/TZ2P) was thus applied further to two representative large-system complexes including porphine dimers and fullerene aggregates to gain a better insight into binding characteristics.

Local nature of substituent effects in stacking interactions

Popular explanations of substituent effects in π-stacking interactions hinge upon substituent-induced changes in the aryl π-system. This entrenched view has been used to explain substituent effects in countless stacking interactions over the past 2 decades. However, for a broad range of stacked dimers, it is shown that substituent effects are better described as arising from local, direct interactions of the substituent with the proximal vertex of the other ring. Consequently, substituent effects in stacking interactions are additive, regardless of whether the substituents are on the same or opposite rings. Substituent effects are also insensitive to the introduction of heteroatoms on distant parts of either stacked ring. This local, direct interaction viewpoint provides clear, unambiguous explanations of substituent effects for myriad stacking interactions that are in accord with robust computational data, including DFT-D and new benchmark CCSD(T) results. Many of these computational results cannot be readily explained using traditional π-polarization-based models. Analyses of stacking interactions based solely on the sign of the electrostatic potential above the face of an aromatic ring or the molecular quadrupole moment face a similar fate. The local, direct interaction model provides a simple means of analyzing substituent effects in complex aromatic systems and also offers simple explanations of the crystal packing of fluorinated benzenes and the recently published dependence of the stability of protein-RNA complexes on the regiochemistry of fluorinated base analogues [J. Am. Chem. Soc.2011, 133, 3687-3689].

A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu

The method of dispersion correction as an add-on to standard Kohn-Sham density functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coefficients and cutoff radii that are both computed from first principles. The coefficients for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination numbers (CN). They are used to interpolate between dispersion coefficients of atoms in different chemical environments. The method only requires adjustment of two global parameters for each density functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of atomic forces. Three-body nonadditivity terms are considered. The method has been assessed on standard benchmark sets for inter- and intramolecular noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean absolute deviations for the S22 benchmark set of noncovalent interactions for 11 standard density functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C(6) coefficients also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in molecules and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems.

Probing substituent effects in aryl-aryl interactions using stereoselective Diels-Alder cycloadditions

Stereoselective Diels-Alder cycloadditions that probe substituent effects in aryl-aryl sandwich complexes were studied experimentally and theoretically. Computations on model systems demonstrate that the stereoselectivity in these reactions is mediated by differential pi-stacking interactions in competing transition states. This allows relative stacking free energies of substituted and unsubstituted sandwich complexes to be derived from measured product distributions. In contrast to gas-phase computations, dispersion effects do not appear to play a significant role in the substituent effects, in accord with previous experiments. The experimental pi-stacking free energies are shown to correlate well with Hammett sigma(m) constants (r = 0.96). These substituent constants primarily provide a measure of the inductive electron-donating and -withdrawing character of the substituents, not donation into or out of the benzene pi-system. The present experimental results are most readily explained using a recently proposed model of substituent effects in the benzene sandwich dimer in which the pi-system of the substituted benzene is relatively unimportant and substituent effects arise from direct through-space interactions. Specifically, these results are the first experiments to clearly show that OMe enhances these pi-stacking interactions, despite being a pi-electron donor. This is in conflict with popular models in which substituent effects in aryl-aryl interactions are modulated by polarization of the aryl pi-system.

Study of the interaction between water and hydrogen sulfide with polycyclic aromatic hydrocarbons

A computational study has been carried out for determining the characteristics of the interaction between one water and hydrogen sulfide molecule with a series of polycyclic aromatic hydrocarbons of increasing size, namely, benzene, anthracene, triphenylene, coronene, circumcoronene, and dicircumcoronene. Potential energy curves were calculated for structures where H(2)X (X=O,S) molecule is located over the central six-membered ring with its hydrogen atoms pointing toward to (mode A) or away from (mode B) the hydrocarbon. The accuracy of different methods has been tested against the results of coupled cluster calculations extrapolated to basis set limit for the smaller hydrocarbons. The spin component scaled MP2 (SCS-MP2) method and a density functional theory method empirically corrected for dispersion (DFT-D) reproduce fairly well the results of high level calculations and therefore were employed for studying the larger systems, though DFT-D seems to underestimate the interaction in hydrogen sulfide clusters. Water complexes in mode A have interaction energies that hardly change with the size of the hydrocarbon due to compensation between the increase in the correlation contribution to the interaction energy and the increase in the repulsive character of the Hartree-Fock energy. For all the other clusters studied, there is a continuous increase in the intensity of the interaction as the size of the hydrocarbon increases, suggesting already converged values for circumcoronene. The interaction energy for water clusters extrapolated to an infinite number of carbon atoms amounts to -13.0 and -15.8 kJ/mol with SCS-MP2 and DFT-D, respectively. Hydrogen sulfide interacts more strongly than water with the hydrocarbons studied, leading to a limiting value of -21.7 kJ/mol with the SCS-MP2 method. Also, complexes in mode B are less stable than the corresponding A structures, with interaction energies amounting to -8.2 and -18.2 kJ/mol for water and hydrogen sulfide, respectively. The DFT-D calculations give values of -16.2 and -9.3 kJ/mol for hydrogen sulfide complexes in modes A and B, less negative than those predicted by the SCS-MP2 method, probably indicating problems with sulfur dispersion parameters.