Kinetic isotope effects for Cl + CH4 ⇌ HCl + CH3 calculated using ab initio semiclassical transition state theory

Calculations were carried out for 25 isotopologues of the title reaction for various combinations of (35)Cl, (37)Cl, (12)C, (13)C, (14)C, H, and D. The computed rate constants are based on harmonic vibrational frequencies calculated at the CCSD(T)/aug-cc-pVTZ level of theory and X(ij) vibrational anharmonicity coefficients calculated at the CCSD(T) /aug-cc-pVDZ level of theory. For some reactions, anharmonicity coefficients were also computed at the CCSD(T)/aug-cc-pVTZ level of theory. The classical reaction barrier was taken from Eskola et al. [J. Phys. Chem. A 2008, 112, 7391-7401], who extrapolated CCSD(T) calculations to the complete basis set limit. Rate constants were calculated for temperatures from ～100 to ～2000 K. The computed ab initio rate constant for the normal isotopologue is in good agreement with experiments over the entire temperature range (～10% lower than the recommended experimental value at 298 K). The ab initio H/D kinetic isotope effects (KIEs) for CH(3)D, CH(2)D(2), CHD(3), and CD(4) are in very good agreement with literature experimental data. The ab initio (12)C/(13)C KIE is in error by ～2% at 298 K for calculations using X(ij) coefficients computed with the aug-cc-pVDZ basis set, but the error is reduced to ～1% when X(ij) coefficients computed with the larger aug-cc-pVTZ basis set are used. Systematic improvements appear to be possible. The present SCTST results are found to be more accurate than those from other theoretical calculations. Overall, this is a very promising method for computing ab initio kinetic isotope effects.     

Ab initio calculation of structure and transport properties of He…X (X = Zn, Cd, Hg) van der Waals complexes

The ground state ab initio CCSD(T) potential curves using various basis sets (aug-cc-pVXZ-PP (X = D, T, Q, 5)) is obtained for the dimers of helium with IIb group metals. The effect of the position of the (mid) bond-functions on the interaction energy is discussed. A Symmetry Adapted Perturbation Theory decomposition of the interaction energy is provided and the trends in the dimer stabilizing and destabilizing contributions are depicted. The spline fitted potential curves are applied together with rigorous statistical formulae in order to obtain the transport coefficients (viscosity coefficients, diffusion coefficients) and the second virial coefficient both for pure constituents and mixtures. The obtained theoretical results are compared with available experimental data. Molecular dynamics is used to obtain reliable values of the diffusion coefficients for all the systems under study.     

Ab initio intermolecular potential energy surface and second pressure virial coefficients of methane

A six-dimensional potential energy hypersurface (PES) for two interacting rigid methane molecules was determined from high-level quantum-mechanical ab initio computations. A total of 272 points for 17 different angular orientations on the PES were calculated utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory with basis sets of aug-cc-pVTZ and aug-cc-pVQZ qualities. The calculated interaction energies were extrapolated to the complete basis set limit. An analytical site-site potential function with nine sites per methane molecule was fitted to the interaction energies. In addition, a semiempirical correction to the analytical potential function was introduced to take into account the effects of zero-point vibrations. This correction includes adjustments of the dispersion coefficients and of a single-parameter within the fit to the measured values of the second virial coefficient B(T) at room temperature. Quantitative agreement was then obtained with the measured B values over the whole temperature range of the measurements. The calculated B values should definitely be more reliable at very low temperatures (T<150 K) than values extrapolated using the currently recommended equation of state.     

Origin of attraction and directionality of the pi/pi interaction: model chemistry calculations of benzene dimer interaction.

A model chemistry for the evaluation of intermolecular interaction between aromatic molecules (AIMI Model) has been developed. The CCSD(T) interaction energy at the basis set limit has been estimated from the MP2 interaction energy near the basis set limit and the CCSD(T) correction term obtained by using a medium size basis set. The calculated interaction energies of the parallel, T-shaped,and slipped-parallel benzene dimers are -1.48, -2.46, and -2.48 kcal/mol, respectively. The substantial attractive interaction in benzene dimer, even where the molecules are well separated, shows that the major source of attraction is not short-range interactions such as charge-transfer but long-range interactions such as electrostatic and dispersion. The inclusion of electron correlation increases attraction significantly. The dispersion interaction is found to be the major source of attraction in the benzene dimer. The orientation dependence of the dimer interaction is mainly controlled by long-range interactions. Although electrostatic interaction is considerably weaker than dispersion interaction, it is highly orientation dependent. Dispersion and electrostatic interactions are both important for the directionality of the benzene dimer interaction.     

Dispersion interactions of carbohydrates with condensate aromatic moieties: theoretical study on the CH-π interaction additive properties

In this article we present the first systematic study of the additive properties (i.e. degree of additivity) of the carbohydrate-aromatic moiety CH-π dispersion interaction. The additive properties were studied on the β-D-glucopyranose, β-D-mannopyranose and α-L-fucopyranose complexes with the naphthalene molecule by comparing the monodentate (single CH-π) and bidentate (two CH-π) complexes. All model complexes were optimized using the DFT-D approach, at the BP/def2-TZVPP level of theory. The interaction energies were refined using single point calculations at highly correlated ab initio methods at the CCSD(T)/CBS level, calculated as E + (E(CCSD(T))-E(MP2))(Small Basis). Bidentate complexes show very strong interactions in the range from -10.79 up to -7.15 and -8.20 up to -6.14 kcal mol(-1) for the DFT-D and CCSD(T)/CBS level, respectively. These values were compared with the sum of interaction energies of the appropriate monodentate carbohydrate-naphthalene complexes. The comparison reveals that the bidentate complex interaction energy is higher (interaction is weaker) than the sum of monodentate complex interaction energies. Bidentate complex interaction energy corresponds to 2/3 of the sum of the appropriate monodentate complex interaction energies (averaging over all modeled carbohydrate complexes). The observed interaction energies were also compared with the sum of interaction energies of the corresponding previously published carbohydrate-benzene complexes. Also in this case the interaction energy of the bidentate complex was higher (i.e. weaker interaction) than the sum of interaction energies of the corresponding benzene complexes. However, the obtained difference is lower than before, while the bidentate complex interaction energy corresponds to 4/5 of the sum of interaction energy of the benzene complexes, averaged over all structures. The mentioned comparison might aid protein engineering efforts where amino acid residues phenylalanine or tyrosine are to be replaced by a tryptophan and can help to predict the changes in the interactions. The observed results also show that DFT-D correctly describes the CH-π interaction energy and their additive properties in comparison to CCSD(T)/CBS calculated interaction energies. Thus, the DFT-D approach might be used for calculation of larger complexes of biological interest, where dispersion interaction plays an important role.     

Intermolecular interactions in nitrogen-containing aromatic systems

π–π and CH···N interactions are vital in biological systems. In this study, stacking and hydrogen-bonded interactions in pyrazine and triazine dimers were investigated by density functional theory combined with symmetry-adapted perturbation theory (DFT-SAPT) and counterpoise (CP)-corrected supermolecular MP2, SCS-MP2, B3LYP-D and CCSD(T) calculations. All interaction energies were computed using the optimized structures at the CP-corrected SCS/aug-cc-pVDZ level, which gave 1–2 kJ/mol lower interaction energies than the ones computed at the MP2 level. For both dimers, doubly hydrogen-bonded and cross-(displaced) stacked orientations were found to be the lowest energy ones. The reference CCSD(T) calculations favored the former structure in both dimer systems, whereas MP2 and SCS-MP2 located the latter as the lowest energy isomer. In particular, the former was found to be lower in energy than the latter by 2.28 and 1.01 kJ/mol at the CCSD(T)/aug-cc-pVDZ level for pyrazine and triazine, respectively. B3LYP-D produced interaction energies in agreement with the CCSD(T) at the equilibrium geometries, but it overestimates them at the short range and underestimates at the long intermonomer separations. Furthermore, it tends to give smaller equilibrium distances compared to the CCSD(T). DFT-SAPT method was in a good agreement with the reference CCSD(T) calculations. This suggests that DFT-SAPT can be employed to compute the full potential energy surface of these dimers. Moreover, DFT-SAPT calculations showed that the electrostatic and dispersion contributions are the most important energy components stabilizing these dimers. The present study aims to show which theoretical method is the most promising one for the investigation of intermolecular interactions dominated by π–π and CH···N. Therefore, the findings obtained in this study can be used to unravel the structures of nucleic acid bases and other systems stabilized by π–π and CH···N interactions.     

Physical nature of silane⋯carbene dimers revealed by state-of-the-art ab initio calculations

Using the SAPT2 + 3(CCD)δMP2 method in complete basis set (CBS) limit, it is shown that the interactions in the recently studied silane⋯carbene dimers are mainly dispersive in nature. Consequently, slow convergence of dispersion energy also forces slow convergence of the interaction energy. Therefore, obtaining very accurate values requires extrapolation of the correlation part to the CBS limit. The most accurate values obtained at the CCSD(T)/CBS level of theory show that the studied silane⋯carbene dimers are rather weakly bound, with interaction energies ranging from about −1.9 to −1.3 kcal/mol. Comparing to CCSD(T)/CBS, it will be shown that SCS-MP2 and MP2C methods clearly underestimate and methods based on SAPT2+ and having some third-order corrections, as well as the MP2 method, overestimate values of interaction energies. Popular SAPT(DFT) method performs better than SCS-MP2 and MP2C; nevertheless, underestimation is still considerable. The underestimation is slightly quenched if third-order dispersion energy and its exchange counterpart is added to the SAPT(DFT). The closest value of CCSD(T)/CBS has been given by the SAPT2 + (3)(CCD)δMP2 method in quadruple-ζ basis set. © 2019 Wiley Periodicals, Inc.     

High-level ab initio computations of structures and interaction energies of naphthalene dimers: origin of attraction and its directionality

The intermolecular interaction energies of naphthalene dimers have been calculated by using an aromatic intermolecular interaction model (a model chemistry for the evaluation of intermolecular interactions between aromatic molecules). The CCSD(T) (coupled cluster calculations with single and double substitutions with noniterative triple excitations) interaction energy at the basis set limit has been estimated from the second-order M?ller-Plesset perturbation interaction energy near saturation and the CCSD(T) correction term obtained using a medium-size basis set. The estimated interaction energies of the set of geometries explored in this work show that two structures emerge as being the lowest energy, and may effectively be considered as isoenergetic on the basis of the errors inherent in out extrapolation procedure. These structures are the slipped-parallel (Ci) structure (-5.73 kcal/mol) and the cross (D2d) structure (-5.28 kcal/mol). The T-shaped (C2v) and sandwich (D2h) dimers are substantially less stable (-4.34 and -3.78 kcal/mol, respectively). The dispersion interaction is found to be the major source of attraction in the naphthalene dimer. The electrostatic interaction is substantially smaller than the dispersion interaction. The large dispersion interaction is the cause of the large binding energies of the cross and slipped-parallel dimers.     

An ab initio study of the electronic structure of BCl3(2+) and its decomposition pathways

Ab initio methods have been used to characterise the lowest energy potential energy surfaces of 1BCl3(2+) and 3BCl3(2+). The methods used are MP2, CCSD, CCSD(T) and MCSCF with 6-311G(d), cc-pVTZ and aug-cc-pVTZ basis sets. While the singlet surface is relatively straight-forward, the triplet surface is very complicated, with many stationary points in close energetic proximity. The singlet surface can fragment to the following products (1BCl + 1Cl+ + 1Cl+), (1Cl+ + 1B+ + 1Cl2), (2BCl+ + 2Cl), while the triplet surface can fragment to (1BCl2+ + 3Cl+) and (2BCl2(2+) + 2Cl). 2BCl2(2+) can further fragment to (1Cl+ + 2BCl+). These results are in good agreement with previous experimental data from coincidence mass spectroscopy. [symbol: see text]1 diagnostic values have been calculated for all of the stationary points of BCl3(2+), using the method of Lee and Taylor. These data, together with CCSD/CCSD(T) energy differences and MCSCF calculations, are used to conclude that most of the stationary points on the singlet surface are well represented using single reference methods. The stationary points of the triplet system have [symbol: see text]1 diagnostic values greater than those for the singlet system, as expected when using the closed-shell [symbol: see text]1 diagnostic method of Lee and Taylor. However, all of the structures have acceptable single reference wavefunctions if the open-shell upper limit of Rienstra-Kiracofe et al. (0.045) is used, a conclusion fully supported by CCSD/CCSD(T) energy differences. CCSD(T) energies determined for the fragmentation asymptotes have been compared with experimental data collated from the NIST Theoretical and Scientific Data website, and the generally very good agreement between theory and experiment reinforces the reliability of the CCSD(T) method.     

Performance of dispersion-corrected density functional theory for the interactions in ionic liquids

Potential energy curves for the dissociation of cation-anion associates representing the building units of ionic liquids have been computed with dispersion corrected DFT methods. Non-local van der Waals density functionals (DFT-NL) for the first time as well as an atom pair-wise correction method (DFT-D3) have been tested. Reference data have been computed at the extrapolated MP2/CBS and estimated CCSD(T)/CBS levels of theory. The investigated systems are combined from two cations (1-butyl-3-methylimidazolium and tributyl(methyl)posphonium) and three anions (chloride, dicyanamide, acetate). We find substantial stabilization from London dispersion energy near equilibrium of 5-7 kcal mol(-1) (about 5-6% of the interaction energy). Equilibrium distances are shortened by 0.03-0.09 ? and fundamental (inter-fragment) vibrational frequencies (which are in the range 140-180 cm(-1)) are increased by typically 10 cm(-1) when dispersion corrections are made. The dispersion-corrected hybrid functional potentials are in general in excellent agreement with the corresponding CCSD(T) reference data (typical deviations of about 1-2%). The DFT-D3 method performs unexpectedly well presumably because of cancellation of errors between the dispersion coefficients of the cations and anions. Due to self-interaction error, semi-local density functionals exhibit severe SCF convergence problems, and provide artificial charge-transfer and inaccurate interaction energies for larger inter-fragment distances. Although these problems may be alleviated in condensed phase simulations by effective Coulomb screening, only dispersion-corrected hybrid functionals with larger amounts of Fock-exchange can in general be recommended for such ionic systems.     

Interactions of CO2 with cluster models of metal–organic frameworks

The interactions between carbon dioxide and cluster models of coordinatively unsaturated metal–organic frameworks (MOFs) were studied using a variety of ab initio methods. Three metal species and three organic linkers in four structures were considered in these models as a representation of the tunable nature of MOFs and the potential multireference character of such systems. Common single-reference methods, such as MP2 and CCSD(T), were compared with multireference methods based on complete active space self-consistent field theory, going as far as multireference configuration interaction with single and double excitations (MRCISD). Special consideration is taken to avoid issues of size inconsistency in the CI results, where an alternate reference is used in the interaction energy definition. The benchmark values are used to judge the adequacy of a selection of density functionals for the current systems. Symmetry-adapted perturbation theory (SAPT) decomposition was performed to elucidate the important effects that comprise the binding interactions. The systems proved to have very limited multireference character, and MP2 values were closer to the CCSD(T) benchmark than the more difficult MRCISD results. Though the SAPT total energies prove to be relatively poor approximations to the benchmark interaction energies, they reveal (in most cases) the correct trends with respect to the choice of the metal. The SAPT energy decompositions indicate that the CO₂ binding is primarily driven by electrostatics, but induction and dispersion also provide sizable, and quite similar, attractive contributions. Importantly, the small diformate model provides a faithful representation of complexes with large aromatic linkers, both in terms of the total interaction energy and the SAPT decomposition.     

The Nature of the Binding of Au, Ag, and Pd to Benzene, Coronene, and Graphene: From Benchmark CCSD(T) Calculations to Plane-Wave DFT Calculations

The adsorption of Ag, Au, and Pd atoms on benzene, coronene, and graphene has been studied using post Hartree-Fock wave function theory (CCSD(T), MP2) and density functional theory (M06-2X, DFT-D3, PBE, vdW-DF) methods. The CCSD(T) benchmark binding energies for benzene-M (M = Pd, Au, Ag) complexes are 19.7, 4.2, and 2.3 kcal/mol, respectively. We found that the nature of binding of the three metals is different: While silver binds predominantly through dispersion interactions, the binding of palladium has a covalent character, and the binding of gold involves a subtle combination of charge transfer and dispersion interactions as well as relativistic effects. We demonstrate that the CCSD(T) benchmark binding energies for benzene-M complexes can be reproduced in plane-wave density functional theory calculations by including a fraction of the exact exchange and a nonempirical van der Waals correction (EE+vdW). Applying the EE+vdW method, we obtained binding energies for the graphene-M (M = Pd, Au, Ag) complexes of 17.4, 5.6, and 4.3 kcal/mol, respectively. The trends in binding energies found for the benzene-M complexes correspond to those in coronene and graphene complexes. DFT methods that use empirical corrections to account for the effects of vdW interactions significantly overestimate binding energies in some of the studied systems.     

Approximations to complete basis set-extrapolated, highly correlated non-covalent interaction energies

The first-principles calculation of non-covalent (particularly dispersion) interactions between molecules is a considerable challenge. In this work we studied the binding energies for ten small non-covalently bonded dimers with several combinations of correlation methods (MP2, coupled-cluster single double, coupled-cluster single double (triple) (CCSD(T))), correlation-consistent basis sets (aug-cc-pVXZ, X = D, T, Q), two-point complete basis set energy extrapolations, and counterpoise corrections. For this work, complete basis set results were estimated from averaged counterpoise and non-counterpoise-corrected CCSD(T) binding energies obtained from extrapolations with aug-cc-pVQZ and aug-cc-pVTZ basis sets. It is demonstrated that, in almost all cases, binding energies converge more rapidly to the basis set limit by averaging the counterpoise and non-counterpoise corrected values than by using either counterpoise or non-counterpoise methods alone. Examination of the effect of basis set size and electron correlation shows that the triples contribution to the CCSD(T) binding energies is fairly constant with the basis set size, with a slight underestimation with CCSD(T)∕aug-cc-pVDZ compared to the value at the (estimated) complete basis set limit, and that contributions to the binding energies obtained by MP2 generally overestimate the analogous CCSD(T) contributions. Taking these factors together, we conclude that the binding energies for non-covalently bonded systems can be accurately determined using a composite method that combines CCSD(T)∕aug-cc-pVDZ with energy corrections obtained using basis set extrapolated MP2 (utilizing aug-cc-pVQZ and aug-cc-pVTZ basis sets), if all of the components are obtained by averaging the counterpoise and non-counterpoise energies. With such an approach, binding energies for the set of ten dimers are predicted with a mean absolute deviation of 0.02 kcal/mol, a maximum absolute deviation of 0.05 kcal/mol, and a mean percent absolute deviation of only 1.7%, relative to the (estimated) complete basis set CCSD(T) results. Use of this composite approach to an additional set of eight dimers gave binding energies to within 1% of previously published high-level data. It is also shown that binding within parallel and parallel-crossed conformations of naphthalene dimer is predicted by the composite approach to be 9% greater than that previously reported in the literature. The ability of some recently developed dispersion-corrected density-functional theory methods to predict the binding energies of the set of ten small dimers was also examined.     

Theoretical reference values for the AE6 and BH6 test sets from explicitly correlated coupled-cluster theory

We propose a new computational protocol to obtain highly accurate theoretical reference data. This protocol employs the explicitly correlated coupled-cluster method with iterative single and double excitations as well as perturbative triple excitations, CCSD(T)(F12), using quadruple-z\zeta basis sets. Higher excitations are accounted for by conventional CCSDT(Q) calculations using double-z\zeta basis sets, while core/core-valence correlation effects are estimated by conventional CCSD(T) calculations using quadruple-z\zeta basis sets. Finally, scalar-relativistic effects are accounted for by conventional CCSD(T) calculations using triple-z\zeta basis sets. In the present article, this protocol is applied to the popular test sets AE6 and BH6. An error analysis shows that the new reference values obtained by our computational protocol have an uncertainty of less than 1 kcal/mol (chemical accuracy). Furthermore, concerning the atomization energies, a cancellation of the basis set incompleteness error in the CCSD(T)(F12) perturbative triples contribution with the corresponding error in the contribution from higher excitations is observed. This error cancellation is diminished by the CCSD(T*)(F12) method. Thus, we recommend the use of the CCSD(T*)(F12) method only for small- and medium-sized basis sets, while the CCSD(T)(F12) approach is preferred for high-accuracy calculations in large basis sets.     

Stabilisation energy of C(6)H(6)...C(6)X(6) (X = F, Cl, Br, I, CN) complexes: complete basis set limit calculations at MP2 and CCSD(T) levels

Stabilisation energies of stacked structures of C(6)H(6)...C(6)X(6) (X = F, Cl, Br, CN) complexes were determined at the CCSD(T) complete basis set (CBS) limit level. These energies were constructed from MP2/CBS stabilisation energies and a CCSD(T) correction term determined with a medium basis set (6-31G**). The former energies were extrapolated using the two-point formula of Helgaker et al. from aug-cc-pVDZ and aug-cc-pVTZ Hartree-Fock energies and MP2 correlation energies. The CCSD(T) correction term is systematically repulsive. The final CCSD(T)/CBS stabilisation energies are large, considerably larger than previously calculated and increase in the series as follows: hexafluorobenzene (6.3 kcal mol(-1)), hexachlorobenzene (8.8 kcal mol(-1)), hexabromobenzene (8.1 kcal mol(-1)) and hexacyanobenzene (11.0 kcal mol(-1)). MP2/SDD** relativistic calculations performed for all complexes mentioned and also for benzene[dot dot dot]hexaiodobenzene have clearly shown that due to relativistic effects the stabilisation energy of the hexaiodobenzene complex is lower than that of hexabromobenzene complex. The decomposition of the total interaction energy to physically defined energy components was made by using the symmetry adapted perturbation treatment (SAPT). The main stabilisation contribution for all complexes investigated is due to London dispersion energy, with the induction term being smaller. Electrostatic and induction terms which are attractive are compensated by their exchange counterparts. The stacked motif in the complexes studied is very stable and might thus be valuable as a supramolecular synthon.     

Ab initio calculations of structures and interaction energies of toluene dimers including CCSD(T) level electron correlation correction

The intermolecular interaction energy of the toluene dimer has been calculated with the ARS-F model (a model chemistry for the evaluation of intermolecular interaction energy between ARomatic Systems using Feller's method), which was formerly called as the AIMI model III. The CCSD(T) (coupled cluster calculations with single and double substitutions with noniterative triple excitations) interaction energy at the basis set limit has been estimated from the second-order Moller-Plesset perturbation interaction energy at the basis set limit obtained by Feller's method and the CCSD(T) correction term obtained using a medium-size basis set. The cross (C(2)) dimer has the largest (most negative) interaction energy (-4.08 kcal/mol). The antiparallel (C(2h)) and parallel (C(S)) dimers (-3.77 and -3.41 kcal/mol, respectively) are slightly less stable. The dispersion interaction is found to be the major source of attraction in the toluene dimer. The dispersion interaction mainly determines the relative stability of the stacked three dimers. The electrostatic interaction of the stacked three dimers is repulsive. Although the T-shaped and slipped-parallel benzene dimers are nearly isoenergetic, the stacked toluene dimers are substantially more stable than the T-shaped toluene dimer (-2.62 kcal/mol). The large dispersion interaction in the stacked toluene dimers is the cause of their enhanced stability.     

Highly Accurate CCSD(T) and DFT–SAPT Stabilization Energies of H‐Bonded and Stacked Structures of the Uracil Dimer

The CCSD(T) interaction energies for the H‐bonded and stacked structures of the uracil dimer are determined at the aug‐cc‐pVDZ and aug‐cc‐pVTZ levels. On the basis of these calculations we can construct the CCSD(T) interaction energies at the complete basis set (CBS) limit. The most accurate energies, based either on direct extrapolation of the CCSD(T) correlation energies obtained with the aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets or on the sum of extrapolated MP2 interaction energies (from aug‐cc‐pVTZ and aug‐cc‐pVQZ basis sets) and extrapolated ΔCCSD(T) correction terms [difference between CCSD(T) and MP2 interaction energies] differ only slightly, which demonstrates the reliability and robustness of both techniques. The latter values, which represent new standards for the H‐bonding and stacking structures of the uracil dimer, differ from the previously published data for the S22 set by a small amount. This suggests that interaction energies of the S22 set are generated with chemical accuracy. The most accurate CCSD(T)/CBS interaction energies are compared with interaction energies obtained from various computational procedures, namely the SCS–MP2 (SCS: spin‐component‐scaled), SCS(MI)–MP2 (MI: molecular interaction), MP3, dispersion‐augmented DFT (DFT–D), M06–2X, and DFT–SAPT (SAPT: symmetry‐adapted perturbation theory) methods. Among these techniques, the best results are obtained with the SCS(MI)–MP2 method. Remarkably good binding energies are also obtained with the DFT–SAPT method. Both DFT techniques tested yield similarly good interaction energies. The large magnitude of the stacking energy for the uracil dimer, compared to that of the benzene dimer, is explained by attractive electrostatic interactions present in the stacked uracil dimer. These interactions force both subsystems to approach each other and the dispersion energy benefits from a shorter intersystem separation.     

Correction to DFT interaction energies by an empirical dispersion term valid for a range of intermolecular distances

The computation of intermolecular interaction energies via commonly used density functionals is hindered by their inaccurate inclusion of medium and long range dispersion interactions. Practical computation of inter- and intra-macrobiomolecule interaction energies, in particular, requires a fairly accurate yet not overly expensive methodology. It is also desirable to compute intermolecular energies not only at their equilibrium (lowest energy) configurations but also over a range of biophysically relevant distances. We present a method to compute intermolecular interaction energies by including an empirical correction for dispersion which is valid over a range of intermolecular distances. This is achieved by optimizing parameters that moderate the empirical correction by explicit comparison of density functional (B3LYP) energies with distance-dependent (DD) reference values obtained at the CCSD(T)/CBS limit. The resulting method, hereafter referred to as B3LYP-DD, yields interaction energies with an accuracy generally better than 1 kcal mol(-1) for different types of noncovalent complexes, over a range of intermolecular distances and interaction strengths, relative to the expensive CCSD(T)/CBS standard. For a training set of dispersion interacting complexes, B3LYP-DD interaction energies in combination with diffuse functions display absolute errors equal to or smaller than 0.68 kcal mol(-1). The empirical correction does not significantly increase the computational cost as compared to standard density functional calculations. Applications relevant to biomolecular energy and structure, such as prediction of DNA base-pair interactions, are also presented.     

Reduced multireference coupled-cluster method: barrier heights for heavy atom transfer, nucleophilic substitution, association, and unimolecular reactions

The recently developed reduced multireference coupled-cluster method with singles and doubles (RMR CCSD) that is perturtatively corrected for triples [RMR CCSD(T)] is employed to compute the forward and reverse barrier heights for 19 non-hydrogen-transfer reactions. The method represents an extension of the conventional single-reference (SR) CCSD(T) method to multireference situations. The results are compared with a benchmark database, which is essentially based on the SR CCSD(T) results. With the exception of seven cases, the RMR CCSD(T) results are almost identical with those based on SR CCSD(T), implying the abatement of MR effects at the SD(T) level relative to the SD level. Using the differences between the RMR CCSD(T) and CCSD(T) barrier heights as a measure of MR effects, modified values for barrier heights of studied reactions are given.     

Theoretical investigation of the binding energies of the iodide ion and xenon atom with decaborane

The interaction of decaborane (B(10)H(14)) with the I(-) ion and the (isoelectronic) Xe atom is investigated using a number of theoretical methods: MP2, CCSD(T), CCSD, spin-orbit CISD, and DFT using the B3LYP, B3PW91, PW91PW91, and PBE0 methods. All non-DFT and some DFT methods agree that B(10)H(14)I(-) is bound by charge-dipole electrostatic forces, charge- and dipole-induced-dipole forces, and dispersion forces, while B(10)H(14)Xe is bound by dipole-induced-dipole forces and dispersion forces. Counterpoise corrections are necessary to obtain reliable results. Relativistic effective core potentials were used for the I, Xe, and B atoms. Basis sets for use with these potentials are discussed as is the question of basis set balance in molecules. We find B(10)H(14)I(-) to be bound by 19.8 kcal/mol and B(10)H(14)Xe by 1.1 kcal/mol, indicating that the charge and polarizability of I(-) play the major role in the interaction energy.