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.     

Comparison of aromatic NH···π, OH···π, and CH···π interactions of alanine using MP2, CCSD, and DFT methods

A comparison of the performance of various density functional methods including long‐range corrected and dispersion corrected methods [MPW1PW91, B3LYP, B3PW91, B97‐D, B1B95, MPWB1K, M06‐2X, SVWN5, ωB97XD, long‐range correction (LC)‐ωPBE, and CAM‐B3LYP using 6‐31+G(d,p) basis set] in the study of CH···π, OH···π, and NH···π interactions were done using weak complexes of neutral (A) and cationic (A⁺) forms of alanine with benzene by taking the Møller–Plesset (MP2)/6‐31+G(d,p) results as the reference. Further, the binding energies of the neutral alanine–benzene complexes were assessed at coupled cluster (CCSD)/6‐31G(d,p) method. Analysis of the molecular geometries and interaction energies at density functional theory (DFT), MP2, CCSD methods and CCSD(T) single point level reveal that MP2 is the best overall performer for noncovalent interactions giving accuracy close to CCSD method. MPWB1K fared better in interaction energy calculations than other DFT methods. In the case of M06‐2X, SVWN5, and the dispersion corrected B97‐D, the interaction energies are significantly overrated for neutral systems compared to other methods. However, for cationic systems, B97‐D yields structures and interaction energies similar to MP2 and MPWB1K methods. Among the long‐range corrected methods, LC‐ωPBE and CAM‐B3LYP methods show close agreement with MP2 values while ωB97XD energies are notably higher than MP2 values. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010     

Theoretical investigation of the cyclic GaO2 and GaS2 molecules at DFT and correlated wave function levels

The geometries and the bonding properties have been predicted for cyclic GaO₂ and GaS₂ species at density functional theory (DFT), MPn (n=2,3,4 with different substitutions), QCISD(T), and CCSD(T) all‐electron correlation levels with 6‐311+G* basis set. The geometrical optimizations and the harmonic vibrational frequency analysis are performed using DFT and second‐order Møller–Plesset (MP2) methods. The relevant energy quantities are also calibrated at the high‐order electron correlation levels [MP3, MP4, quadratic configuration interaction (QCI), and coupled cluster (CC)]. Each species possesses a ²A₂ ground state with a higher energy level ²A₁ state. The corresponding state–state separations are about 32 kcal/mol for GaO₂ species and about 20 kcal/mol for GaS₂ species at the QCISD(T)/6‐311+G* level. The QCISD(T) and CCSD(T) calculations yield dissociation energies of 42.0 and 59.0 kcal/mol for two species, respectively, and other methods yield dissociation energies within ∼5 kcal/mol. Result analysis has indicated that the cyclic GaO₂ should be classified as superoxide and the GaS₂ species should be classified as supersulfide in their ground state, and those in the excited state (²A₁) should not be. However, the cyclic GaS₂ (²A₂) is less ionic than the GaO₂ (²A₂) and they are far less ionic than NaO₂. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 222–231, 2001     

Characterization of the potential energy surfaces of two small but challenging noncovalent dimers: (P2)2 and (PCCP)2

This work characterizes eight stationary points of the P₂ dimer and six stationary points of the PCCP dimer, including a newly identified minimum on both potential energy surfaces. Full geometry optimizations and corresponding harmonic vibrational frequencies were computed with the second‐order Møller–Plesset (MP2) electronic structure method and six different basis sets: aug‐cc‐pVXZ, aug‐cc‐pV(X+d)Z, and aug‐cc‐pCVXZ where X = T, Q. A new L‐shaped structure with C₂ symmetry is the only minimum for the P₂ dimer at the MP2 level of theory with these basis sets. The previously reported parallel‐slipped structure with C_2h symmetry and a newly identified cross configuration with D₂ symmetry are the only minima for the PCCP dimer. Single point energies were also computed using the canonical MP2 and CCSD(T) methods as well as the explicitly correlated MP2‐F12 and CCSD(T)‐F12 methods and the aug‐cc‐pVXZ (X = D, T, Q, 5) basis sets. The energetics obtained with the explicitly correlated methods were very similar to the canonical results for the larger basis sets. Extrapolations were performed to estimate the complete basis set (CBS) limit MP2 and CCSD(T) binding energies. MP2 and MP2‐F12 significantly overbind the P₂ and PCCP dimers relative to the CCSD(T) and CCSD(T)‐F12 binding energies by as much as 1.5 kcal mol^?1 for the former and 5.0 kcal mol^?1 for the latter at the CBS limit. The dominant attractive component of the interaction energy for each dimer configuration was dispersion according to several symmetry‐adapted perturbation theory analyses. © 2014 Wiley Periodicals, Inc.     

Accurate potential energy curves for the group 12 dimers Zn2, Cd2, and Hg2

Potential energy curves of the electronic ground states of the group 12 dimers Zn₂ and Cd₂ were computed at the CCSD(T) level of theory, including full triple corrections $\Updelta$ T in the coupled-cluster procedure, and spin-orbit (SO) contributions from four-component coupled-cluster calculations, extrapolated to the complete basis set (CBS) limit. For Hg₂, the potential energy curve published recently (Pahl et al. in J Chem Phys 132:114301, 2010] is complemented in this work by non-relativistic calculations to quantify and discuss relativistic effects. We obtain very accurate fits of our CBS/CCSD(T) and CBS/CCSD(T)+ $\Updelta$ T data points to an analytically simple and computationally efficient extended Lennard Jones form. For the CBS/CCSD(T)+ $\Updelta$ T+SO curves, we obtain dissociation energies of D _e?=?226?cm^?1 and D _e?=?319?cm^?1 for Zn₂ and Cd₂ respectively, in very good agreement with recent theoretical calculations and experimental data. We also present equilibrium distances and rotational and vibrational spectroscopic constants to compare with available theoretical and experimental data. The results obtained for non-relativistically treated Hg₂ continue nicely the trends with increasing atom number preset by Zn₂ and Cd₂, confirming that indeed, relativistic effects account for the known peculiarities for the mercury dimer.     

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.     

Intermolecular potential energy surface for CS2 dimer

A new four‐dimensional intermolecular potential energy surface for CS₂ dimer is obtained by ab initio calculation of the interaction energies for a range of configurations and center‐of‐mass separation distances for the first time. The calculations were performed using the supermolecular approach at the Møller–Plesset second‐order perturbation (MP2) level of theory with the augmented correlation consistent basis sets (aug‐cc‐pVxZ, x = D, T) and corrected for the basis‐set superposition error using the full counterpoise correction method. A two‐point extrapolation method was used to extrapolate the calculated energy points to the complete basis set limit. The effect of using the higher levels of theory, quadratic configuration interaction containing single, double, and perturbative triple excitations QCISD(T) and coupled cluster singles, doubles and perturbative triples excitations CCSD(T), on the shape of potential energy surface was investigated. It is shown that the MP2 level of theory apparently performs extremely poorly for describing the intermolecular potential energy surface, overestimating the total energy by a factor of nearly 1.73 in comparison with the QCISD(T) and CCSD(T) values. The value of isotropic dipole–dipole dispersion coefficient (C₆) of CS₂ fluid was obtained from the extrapolated MP2 potential energy surface. The MP2 extrapolated energy points were fitted to well‐known analytical potential functions using two different methods to represent the potential energy surface analytically. The most stable configuration of the dimer was determined at R = 6.23 au, α = 90°, β = 90°, and γ = 90°, with a well depth of 3.980 kcal mol^?1 at the MP2 level of theory. Finally, the calculated second virial coefficients were compared with experimental values to test the quality of the presented potential energy surface. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011.     

Nature of closed‐ and open‐shell interactions between noble metals and rare gas atoms

Interactions between noble metals and rare gases have become an interesting topic over the last few years. In this work, a computational study of the open‐shell (d¹⁰s¹) and closed‐shell (d¹⁰s and d¹⁰s²) noble metals (M = Cu, Ag, and Au) with three heaviest rare gas atoms (Rg = Kr, Xe, and Rn) has been performed. Potential energy curves based on ab initio [MP2, MP4, QCISD, and CCSD(T)] and DFT functionals (M06‐2X and CAM‐B3LYP) were obtained for ionic and neutral AuXe complexes. Dissociation energies indicate that neutral metals have the lowest and cationic metals have the highest affinities for interaction with rare gas atoms. For the same metals, there is a continuous increase in dissociation energies (D_e) from Kr to Rn. The nature of bonding and the trend of D_e and equilibrium bond lengths (R_e) have been interpreted by means of quantum theory of atoms in molecules, natural bond orbital, and energy decomposition analysis. © 2013 Wiley Periodicals, Inc.     

Intermolecular Interactions in Weak Donor–Acceptor Complexes from Symmetry‐Adapted Perturbation and Coupled‐Cluster Theory: Tetracyanoethylene–Benzene and Tetracyanoethylene–p‐Xylene

The interactions in the complexes of tetracyanothylene (TCNE) with benzene and p‐xylene, often classified as weak electron donor–acceptor (EDA) complexes, are investigated by a range of quantum chemical methods including intermolecular perturbation theory at the DFT‐SAPT (symmetry‐adapted perturbation theory combined with density functional theory) level and explicitly correlated coupled‐cluster theory at the CCSD(T)‐F12 level. The DFT‐SAPT interaction energies for TCNE–benzene and TCNE–p‐xylene are estimated to be ?35.7 and ?44.9 kJ mol^?1, respectively, at the complete basis set limit. The best estimates for the CCSD(T) interaction energy are ?37.5 and ?46.0 kJ mol^?1, respectively. It is shown that the second‐order dispersion term provides the most important attractive contribution to the interaction energy, followed by the first‐order electrostatic term. The sum of second‐ and higher‐order induction and exchange–induction energies is found to provide nearly 40 % of the total interaction energy. After addition of vibrational, rigid‐rotor, and translational contributions, the computed internal energy changes on complex formation approach results from gas‐phase spectrophotometry at elevated temperatures within experimental uncertainties, while the corresponding entropy changes differ substantially.     

Theoretical study in [C2H4–Tl]+ and [C2H2–Tl]+ complexes

We studied the attraction between [C₂H_n] and Tl(I) in the hypothetical [C₂H_n–Tl]⁺ complexes (n = 2,4) using ab initio methodology. We found that the changes around the equilibrium distance C–Tl and in the interaction energies are sensitive to the electron correlation potential. We evaluated these effects using several levels of theory, including Hartree–Fock (HF), second‐order Møller–Plesset (MP2), MP4, coupled cluster singles and doubles CCSD(T), and local density approximation augmented by nonlocal corrections for exchange and correlation due to Becke and Perdew (LDA/BP). The obtained interaction energies differences at the equilibrium distance R_e (C–Tl) range from 33 and 46 kJ/mol at the different levels used. These results indicate that the interaction between olefinic systems and Tl(I) are a real minimum on the potential energy surfaces (PES). We can predict that these new complexes are viable for synthesizing. At long distances, the behavior of the [C₂H_n]–Tl⁺ interaction may be related mainly to charge‐induced dipole and dispersion terms, both involving the individual properties of the olefinic π‐system and thallium ion. However, the charge‐induced dipole term (R^?4) is found as the principal contribution in the stability at long and short distances. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Structure and Stability of Complexes of <Emphasis Type="Italic">N,N</Emphasis>′-Di(9-anthrylmethyl)-1,2-diaminoethane with Cations of Metals from IIB group: Quantum-chemical Study

Structural and energy characteristics of N,N′-di(9-anthrylmethyl)-1,2-diaminoethane complexes with Z²⁺, Cd²⁺, and Hg²⁺ cations were investigated applying the method DFT (B3LYP/LanL2DZ). The interaction of the diamine with the metal cations results in formation of complexes with various types of structures containing a di-, tri-, or tetracoordinated metal, the latter complexes being the most stable. In all types of complexes the energy of complexing decreases in the series Zn²⁺, Hg²⁺, Cd²⁺ and is determined by combined effect of steric and orbital factors.     

DFT study on the hydrogen bonds of phenol–cyclohexanone and phenol–H2O2 in the Baeyer–Villiger oxidation

Hydrogen bonds of phenol–cyclohexanone and phenol–H₂O₂ in the studied Baeyer–Villiger (B–V) oxidation have been investigated by HF, B3LYP, and MP2 methods with various basis sets. The accurate single‐point energies were performed using CCSD(T)/6‐31+G(d,p) and CCSD(T)/aug‐cc‐pVDZ on the optimized geometries of MP2/6‐31+G(d,p). It has been confirmed that B3LYP/6‐31+G(d,p) could be used to study such hydrogen bonds. Energetic analysis of complexes was carried out using the Xantheas method with BSSE corrected by CP method. Orbital energy order (ε) illuminated that phenol with good hydrogen donor‐acceptor property can interact with cyclohexanone or H₂O₂ to form hydrogen bound complexes, and the binding energies (BE) range from ?4.38 to ?14.06 kcal mol^?1. NBO analysis indicated that the redistribution of atomic charges in the complexes facilitated nucleophilic attack of H₂O₂ on cyclohexanone. The calculated results match remarkably well with the experimental phenomena. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Benchmarking density functionals in conjunction with Grimme's dispersion correction for noble gas dimers (Ne2, Ar2, Kr2, Xe2, Rn2)

Eleven exchange‐correlational functionals of different types corrected for dispersion by Grimme's D3 correction in conjunction with the aug‐cc‐pVTZ basis set were tested on the following noble gas (Ng) dimers: Ne₂, Ar₂, Kr₂, Xe₂, and Rn₂. For comparison, the D2 and D3BJ corrections were probed with the B3LYP functional. From post‐HF wavefunction methods, CCSD(T) theory was also included. The investigated properties involved potential energy curves, equilibrium bond distances, and interaction energies. The B3LYP‐D3, B3LYP‐D3BJ, and PBE0‐D3 functionals performed overall best for bond distances, while B3LYP‐D3 and B97‐D3 performed best for interaction energies. The importance of fortunate error cancellations was seen in the often reduced agreement with reference data upon correction for BSSE. As several functionals performed well selectively for some noble gases (and poorly for others), we also analysed the performance on the Ng₂ dimers individually and recommended DFT‐D3 functionals for the calculation of large clusters of each Ng.     

A quantum mechanical approach to the kinetics of the hydrogen abstraction reaction H2O2 + •OH → HO2 + H2O

The kinetics of the hydrogen abstraction from H₂O₂ by ^?OH has been modeled with MP2/6‐31G*//MP2/6‐31G*, MP2‐SAC//MP2/6‐31G*, MP2/6‐31+G**//MP2/6‐31+G**, MP2‐SAC// MP2/6‐31+G**, MP4(SDTQ)/6‐311G**//MP2/6‐31G*, CCSD(T)/6‐31G*//CCSD(T)/6‐31G*, CCSD(T)/6‐31G**//CCSD(T)/6‐31G**, CCSD(T)/6‐311++G**//MP2/6‐31G* in the gas phase. MD simulations have been used to generate initial geometries for the stationary points along the potential energy surface for hydrogen abstraction from H₂O₂. The effective fragment potential (EFP) has been used to optimize the relevant structures in solution. Furthermore, the IEFPCM model has been used for the supermolecules generated via MD calculations. IEFPCM/MP2/6‐31G* and IEFPCM/CCSD(T)/6‐31G* calculations have also been performed for structures without explicit water molecules. Experimentally, the rate constant for hydrogen abstraction by ^?OH drops from 1.75 × 10^?12 cm³ molecule^?1 s^?1 in the gas phase to 4.48 × 10^?14 cm³ molecule^?1 s^?1 in solution. The same trend has been reproduced best with MP4 (SDTQ)/6‐311G**//MP2/6‐31G* in the gas phase (0.415 × 10^?12 cm³ molecule^?1 s^?1) and with EFP (UHF/6‐31G*) in solution (3.23 × 10^?14 cm³ molecule^?1 s^?1). © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 502–514, 2005     

Density functional study of guanine and uracil quartets and of guanine quartet/metal ion complexes

The structures and interaction energies of guanine and uracil quartets have been determined by B3LYP hybrid density‐functional calculations. The total interaction energy ΔE^T of the C_4h‐symmetric guanine quartet consisting of Hoogsteen‐type base pairs with two hydrogen bonds between two neighbor bases is −66.07 kcal/mol at the highest level. The uracil quartet with C6 H6O4 interactions between the individual bases has only a small interaction energy of −20.92 kcal mol⁻¹, and the interaction energy of −24.63 kcal/mol for the alternative structure with N3 H3O4 hydrogen bonds is only slightly more negative. Cooperative effects contribute between 10 and 25% to all interaction energies. Complexes of metal ions with G‐quartets can be classified into different structure types. The one with Ca²⁺ in the central cavity adopts a C_4h‐symmetric structure with coplanar bases, whereas the energies of the planar and nonplanar Na⁺ complexes are almost identical. The small ions Li⁺, Be²⁺, Cu⁺, and Zn²⁺ prefer a nonplanar S₄‐symmetric structure. The lack of coplanarity prevents probably a stacking of these base quartets. The central cavity is too small for K⁺ ions and, therefore, this ion favors in contrast to all other investigated ions a C₄‐symmetric complex, which is 4.73 kcal/mol more stable than the C_4h‐symmetric one. The distance 1.665 Å between K⁺ and the root‐mean‐square plane of the guanine bases is approximately half of the distance between two stacked G‐quartets. The total interaction energy of alkaline earth ion complexes exceeds those with alkali ions. Within both groups of ions the interaction energy decreases with an increasing row position in the periodic table. The B3LYP and BLYP methods lead to similar structures and energies. Both methods are suitable for hydrogen‐bonded biological systems. Compared with the before‐mentioned methods, the HCTH functional leads to longer hydrogen bonds and different relative energies for two U‐quartets. Finally, we calculated also structures and relative energies with the MMFF94 forcefield. Contrary to all DFT methods, MMFF94 predicts bifurcated C HO contacts in the uracil quartet. In the G‐quartet, the MMFF94 hydrogen bond distances N2 H22N7 are shorter than the DFT distances, whereas the N1 H1O6 distances are longer. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 109–124, 2001     

Three-dimensional potential energy surface of selected carbohydrates' CH/π dispersion interactions calculated by high-level quantum mechanical methods

In this study we present the first systematic computational three‐dimensional scan of carbohydrate hydrophobic patches for the ability to interact through CH/π dispersion interactions. The carbohydrates β‐d‐ glucopyranose, β‐d‐ mannopyranose and α‐l‐ fucopyranose were studied in a complex with a benzene molecule, which served as a model of the CH/π interaction in carbohydrate/protein complexes. The 3D relaxed scans were performed at the SCC‐DFTB‐D level with 3 757 grid points for both carbohydrate hydrophobic sides. The interaction energy of all grid points was recalculated at the DFT‐D BP/def2‐TZVPP level. The results obtained clearly show highly delimited and separated areas around each CH group, with an interaction energy up to ?5.40 kcal mol^?1. The results also show that with increasing H???π distance these delimited areas merge and form one larger region, which covers all hydrogen atoms on that specific carbohydrate side. Simultaneously, the interaction becomes weaker with an energy of ?2.5 kcal mol^?1. All local energy minima were optimized at the DFT‐D BP/def2‐TZVPP level and the interaction energies of these complexes were refined by use of the high‐level ab initio computation at the CCSD(T)/CBS level. Results obtained from the optimization suggest that the CH group hydrogen atoms are not equivalent and the interaction energy at the CCSD(T)/CBS level range from ?3.54 to ?5.40 kcal mol^?1. These results also reveal that the optimal H???π distance for the CH/π dispersion interaction is approximately (2.310±0.030) Å, and the angle defined as carbon‐hydrogen‐benzene geometrical centre is (180±30)°. These results reveal that whereas the dispersion interactions with the lowest interaction energies are quite strictly located in space, the slightly higher interaction energy regions adopt a much larger space.     

A post‐HF study on the halogen bonding interaction of pyrene with diatomic halogen molecules

We present a detailed SCS‐MP2 study on the potential energy curves (PEC) for interactions between diatomic halogen molecules and pyrene. BSSE corrected CCSD[T] energies at equilibrium distances are computed and compared to CCSD(T) energies. The most stable conformation of these weakly bound van der Waals complexes is almost linear in the perpendicular direction to the pyrene plane. The complexes of highly polarizable bromine and iodine molecules with pyrene are very stable and they carry rather large number of vibrational states. Despite its small size, F₂ also forms strong halogen bonding similar to Br₂ and I₂. The interaction between Cl₂ and pyrene is the weakest and it is attributed to the highest polarizability / molar mass ratio of chlorine among the others. I₂‐pyrene is found to be the most stable complex due to the strongest mutual polarization effects and is carrying more than 60 vibrational states. Due to the rather large number of electrons in some complexes, the relativistic corrections are also considered. © 2016 Wiley Periodicals, Inc.     

Regioselectivity in cycloaddition reaction between phosphaacetylene and diazomethane: An ab initio study

The [3 + 2] cycloaddition reaction of phosphaacetylene with diazomethane was investigated by means of high level ab initio calculations. It was deduced that the aromatic diazaphosphole is formed via a nonaromatic intermediate. The regiospecificity of the reaction is thus determined by the energy difference between the two transition states that lead to the two possible regioisomeric intermediates. Of the transition states in the concerted pathways, the one leading to the regioisomer with two PC bonds ( 3 ) was found to be more stable at all the levels of theory investigated, including coupled-cluster singles doubles (CCSD)(T)/6-311 + G^*//Møller-Plessett(MP)2/6-311 + G^* (+ basis set superposition, BSSE, correction). The energy difference between the two transition states, however, is always less than 2 kcal/mol. When the free energies in the two reactions are calculated by use of the harmonic frequencies, the energy separation between the two transition structures remains practically unchanged. The free energy of activation ΔG^† was 21 kcal/mol at the CCSD(T)/6-311 + G^* level of theory and use of the MP2/6-31 + G^* frequencies. At the MP2 level, a rather stable complex is obtained in the initial phase of the reaction. However, the stability of the complexes decreases at the CCSD(T) level, and application of the BSSE correction results in unstable complexes. © 1997 by John Wiley & Sons, Inc.     

On geometries of stacked and H-bonded nucleic acid base pairs determined at various DFT, MP2, and CCSD(T) levels up to the CCSD(T)/complete basis set limit level

The geometries and interaction energies of stacked and hydrogen-bonded uracil dimers and a stacked adeninecdots, three dots, centeredthymine pair were studied by means of high-level quantum chemical calculations. Specifically, standard as well as counterpoise-corrected optimizations were performed at second-order Moller-Plesset (MP2) and coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)] levels with various basis sets up to the complete basis set limit. The results can be summarized as follows: (i) standard geometry optimization with small basis set (e.g., 6-31G(*)) provides fairly reasonable intermolecular separation; (ii) geometry optimization with extended basis sets at the MP2 level underestimates the intermolecular distances compared to the reference CCSD(T) results, whereas the MP2/cc-pVTZ counterpoise-corrected optimization agrees well with the reference geometries and, therefore, is recommended as a next step for improving MP2/cc-pVTZ geometries; (iii) the stabilization energy of stacked nucleic acids base pairs depends considerably on the method used for geometry optimization, so the use of reliable geometries, such as counterpoise-corrected MP2/cc-pVTZ ones, is recommended; (iv) the density functional theory methods fail completely in locating the energy minima for stacked structures and when the geometries from MP2 calculations are used, the resulting stabilization energies are strongly underestimated; (v) the self-consistent charges-density functional tight binding method, with inclusion of the empirical dispersion energy, accurately reproduces interaction energies and geometries of dispersion-bonded (stacked) complexes; this method can thus be recommended for prescanning the potential energy surfaces of van der Waals complexes.     

Assessment of ab initio MP2 and density functionals for characterizing the potential energy profiles of the SN2 reactions at N center

The potential energy profiles of five selected bimolecular nucleophilic substitution (S_N2) reactions at nitrogen (N) center have been reinvestigated with the CCSD(T), G3[MP2,CCSD(T)], MP2, and some density functional methods. The basis sets of 6‐31+G(d,p) and 6‐311+G(3d,2p) are used for the MP2 and density functional calculations. Taking the relative energies at the CCSD(T)/CBS level of theory as benchmarks, we recommend the MP2, B97‐K, B2K‐PLYP, BMK, ωB97X‐D, M06‐2X, M05‐2X, CAM‐B3LYP, M08‐SO, and ωB97X methods to generally characterize the potential energy profiles for the S_N2 reactions at N center. Furthermore, these recommended methods with the relatively small 6‐31+G(d,p) basis set may also be used to perform direct classical trajectory simulations to uncover the dynamic behaviors of the S_N2 reactions at N center. © 2012 Wiley Periodicals, Inc.