Explicitly correlated benchmark calculations on C8H8 isomer energy separations: how accurate are DFT,double-hybrid,and composite ab initio procedures?

Accurate isomerization energies are obtained for a set of 45 C₈H₈ isomers by means of the high-level, ab initio W1-F12 thermochemical protocol. The 45 isomers involve a range of hydrocarbon functional groups, including (linear and cyclic) polyacetylene, polyyne, and cumulene moieties, as well as aromatic, anti-aromatic, and highly-strained rings. Performance of a variety of DFT functionals for the isomerization energies is evaluated. This proves to be a challenging test: only six of the 56 tested functionals attain root mean square deviations (RMSDs) below 3?kcal?mol^?1 (the performance of MP2), namely: 2.9 (B972-D), 2.8 (PW6B95), 2.7 (B3PW91-D), 2.2 (PWPB95-D3), 2.1 (ωB97X-D), and 1.2 (DSD-PBEP86) kcal?mol^?1. Isomers involving highly-strained fused rings or long cumulenic chains provide a ‘torture test’ for most functionals. Finally, we evaluate the performance of composite procedures (e.g. G4, G4(MP2), CBS-QB3, and CBS-APNO), as well as that of standard ab initio procedures (e.g. MP2, SCS-MP2, MP4, CCSD, and SCS-CCSD). Both connected triples and post-MP4 singles and doubles are important for accurate results. SCS-MP2 actually outperforms MP4(SDQ) for this problem, while SCS-MP3 yields similar performance as CCSD and slightly bests MP4. All the tested empirical composite procedures show excellent performance with RMSDs below 1?kcal?mol^?1.     

A high level computational study of the CH4/CF4 dimer: how does it compare with the CH4/CH4 and CF4/CF4 dimers?

The interaction within the methane–methane (CH₄/CH₄), perfluoromethane–perfluoromethane (CF₄/CF₄) methane–perfluoromethane dimers (CH₄/CF₄) was calculated using the Hartree–Fock (HF) method, multiple orders of Møller–Plesset perturbation theory [MP2, MP3, MP4(DQ), MP4(SDQ), MP4(SDTQ)], and coupled cluster theory [CCSD, CCSD(T)], as well as the PW91, B97D, and M06-2X density functional theory (DFT) functionals. The basis sets of Dunning and coworkers (aug-cc-pVxZ, x?=?D, T, Q), Krishnan and coworkers [6-311++G(d,p), 6-311++G(2d,2p)], and Tsuzuki and coworkers [aug(df, pd)-6-311G(d,p)] were used. Basis set superposition error (BSSE) was corrected via the counterpoise method in all cases. Interaction energies obtained with the MP2 method do not fit with the experimental finding that the methane–perfluoromethane system phase separates at 94.5?K. It was not until the CCSD(T) method was considered that the interaction energy of the methane–perfluoromethane dimer (?0.69?kcal?mol^?1) was found to be intermediate between the methane (?0.51?kcal?mol^?1) and perfluoromethane (?0.78?kcal?mol^?1) dimers. This suggests that a perfluoromethane molecule interacts preferentially with another perfluoromethane (by about 0.09?kcal?mol^?1) than with a methane molecule. At temperatures much lower than the CH₄/CF₄ critical solution temperature of 94.5?K, this energy difference becomes significant and leads perfluoromethane molecules to associate with themselves, forming a phase separation. The DFT functionals yielded erratic results for the three dimers. Further development of DFT is needed in order to model dispersion interactions in hydrocarbon/perfluorocarbon systems.     

A computational characterization of N2@C60

Recent observations of N₂@C₆₀ are supported computationally. The geometry is optimized at the B3LYP/3-21G and PW91/3-21G levels. The lowest-energy structure has the N₂ unit oriented towards a pair of parallel pentagons so that the complex exhibits D_5d symmetry. Single-point energy calculations are further carried out at the B3LYP/6-31G*, PW91/6-31G* and MP2?=?FC/6-31G* levels and corrected for the basis set superposition error (BSSE). The MP2?=?FC/6-31G* treatment with the BSSE correction gives a stabilization energy of -9.3?kcal?mol^?1, whereas DFT approaches mostly fail to produce a stabilization. The entropy term for the encapsulation is also evaluated and leads to a standard Gibbs energy change upon encapsulation at room temperature of -3.3?kcal?mol^?1. The computed structural and vibrational characteristics are also reported.     

Theoretical study of rearrangements in water dimer and trimer

The global minimum and transition states for the acceptor-tunnelling, donor-acceptor interchange and bifurcation tunnelling rearrangements of the water dimer, and the single-flip, bifurcation and concerted proton transfer processes in the water trimer have been reinvestigated. Our analysis of the tunnelling splittings and spectroscopy is based on ab initio calculations at the computational level of second-order M?ller-Plesset (MP2) theory with basis sets of aug-cc-pVXZ quality (X = D, T, Q for the dimer; X = D, T for the trimer). In both water dimer and trimer, the binding energy, barrier heights, intermonomer distances, and harmonic frequencies converge smoothly as the size of the basis set increases. In the water dimer, the binding energy was evaluated as 5.09kcal mol^?1, while the activation energies are 0.52 (acceptor-tunnelling) 0.79 (donor-acceptor interchange), and 1.94 kcal mol^?1 (bifurcation tunnelling) at the MP2/aug-cc-pVQZ level. In the water trimer, the binding energy was evaluated as 16.29 kcal mol^?1, while the activation energies are 0.28 (single-flip), 2.34 (bifurcation), and 26.36 (proton transfer) kcal mol^?1 at the MP2/aug-cc-pVTZ level.     

The keto-enol equilibrium in substituted acetaldehydes: focal-point analysis and ab initio limit

High-level ab initio electronic structure calculations up to the CCSD(T) theory level, including extrapolations to the complete basis set (CBS) limit, resulted in high precision energetics of the tautomeric equilibrium in 2-substituted acetaldehydes (XH₂C-CHO). The CCSD(T)/CBS relative energies of the tautomers were estimated using CCSD(T)/aug-cc-pVTZ, MP3/aug-cc-pVQZ, and MP2/aug-cc-pV5Z calculations with MP2/aug-cc-pVTZ geometries. The relative enol (XHC?=?CHOH) stabilities (ΔE _{e,CCSD(T)/CBS}) were found to be 5.98?±?0.17, ?1.67?±?0.82, 7.64?±?0.21, 8.39?±?0.31, 2.82?±?0.52, 10.27?±?0.39, 9.12?±?0.18, 5.47?±?0.53, 7.50?±?0.43, 10.12?±?0.51, 8.49?±?0.33, and 6.19?±?0.18?kcal?mol^?1 for X?=?BeH, BH₂, CH₃, Cl, CN, F, H, NC, NH₂, OCH₃, OH, and SH, respectively. Inconsistencies between the results of complex/composite energy computations methods Gn/CBS (G2, G3, CBS-4M, and CBS-QB3) and high-level ab initio methods (CCSD(T)/CBS and MP2/CBS) were found. DFT/aug-cc-pVTZ results with B3LYP, PBE0 (PBE1PBE), TPSS, and BMK density functionals were close to the CCSD(T)/CBS levels (MAD?=?1.04?kcal?mol^?1).     

Atomic additivity of the correlation energy in molecules—an ab initio MP4 and G3 study

It is shown that the closed shell valence electron molecular correlation energy of organic molecules in their ground states is a homogeneous multilinear function of the numbers of neutral atoms in their canonical hybridization state. The additivity is a robust feature, which holds for MP2(fc), MP3(fc) and MP4(fc) model calculations. The latter results obtained on a test set of 91 widely different organic molecules, exhibiting a whole gamut of electronic structure patterns, are excellent as evidenced by the absolute average deviation from the additivity values (AAD) of only 1.4 kcal mol^?1 and R ² = 0.999 93. The maximum absolute deviation (MAD) is 5.3 kcal mol^?1. The additivity formula for the total molecular electron correlation retrieved from G3 calculations also has an excellent performance (AAD = 1.2 kcal mol^?1, R ² = 0.999 98 and MAD = 7.2 kcal mol^?1). If it is taken into account that the additivity formulae require only back of the envelope calculations, these results are remarkable indeed, in particular since the G3 correlation energies span a very large range from 180.7 (methane) to 1642.8 (hexafluorocyclopropane) kcal mol^?1. Comparison of the exact electron correlation energies in free atoms with the corresponding average correlation energies in molecules reveals that a substantial increase in the latter provides an important contribution in overcoming a very strong Coulomb repulsion between the nuclei. It is shown that the additivity formulae are useful in detecting some special molecular features such as strong resonance and anti-aromaticity.     

Mass spectrometric and ab initio study of the interaction between 9-methylguanine and amino acid amide group

The temperature dependent field ionization mass spectrometry method combined with ab initio calculations was used to determine the interaction energies and the structures of 9-methylguanine-acrylamide dimers. Acrylamide mimics the side chain amide group of the natural amino acids asparagine and glutamine. The experimental enthalpy of the dimer formation derived from the van't Hoff plot is ?59.5 ± 3.8 kJ mol^?1. The value is higher than interaction energies between acrylamide and other nucleic acid bases which were determined to be ?57.0 for 1-methylcytosine, ?52.0 for 9-methyladenine, and ?40.6 kJ mol^?1 for 1-methyl-uracil. In total, eight hydrogen bonded dimers formed by the three lowest energy 9-methylguanine tautomers and acrylamide were found in the quantum chemical calculations performed at the DFT/B3LYP/6-31++G?? and MP2/6-31++G?? levels of theory. The relative stability and the interaction energies of the dimers were calculated accounting for the basis set superposition error and the zero-point vibrational energy correction. The lowest energy dimer found in the calculations is formed by acrylamide (Ac) with the keto tautomer of 9-methylguanine (Gk). It is stabilized by two intermolecular H bonds, C6=O(Gk) · · · H—N(Ac) and Nl—H(Gk) · · ·O(Ac), and it is more stable than the second lowest energy dimer by ≈ 25 kJ mol^?1. The calculated interaction energies of the lowest energy 9-methylguanine-acrylamide dimer are ?65.0 kJ mol^?1 and ?67.7 kJ mol^?1 at the MP2 and DFT levels of theory, respectively. The experimental enthalpy of the dimer formation is in good agreement with both the calculated interaction energies of the GkAc dimer and much higher than the interaction energies calculated for all other 9-methylguanine-acrylamide dimers. This proved that only one dimer was present in the experimental samples. To verify whether acrylamide is a good model of the amino acid-amide group, we performed direct calculations of the 9-methylguanine-glutamine dimers at the same levels of theory as used for the complexes involving acrylamide. The interaction energies found for the lowest energy 9-methylguanine-glutamine dimer are ?65.1 kJ mon^?1 (MP2/6-31++G??) and ?66.2 kJ mol^?1 (DFT/B3LYP/6-31++G??) and these values are very close (within 0.5 kJ mol^?1) to the interaction energies obtained for the 9-methylguanine-acrylamide dimers.     

Enthalpy difference between conformations of normal alkanes: effects of basis set and chain length on intramolecular basis set superposition error

The quantum chemistry of conformation equilibrium is a field where great accuracy (better than 100?cal?mol^?1) is needed because the energy difference between molecular conformers rarely exceeds 1000–3000?cal?mol^?1. The conformation equilibrium of straight-chain (normal) alkanes is of particular interest and importance for modern chemistry. In this paper, an extra error source for high-quality ab initio (first principles) and DFT calculations of the conformation equilibrium of normal alkanes, namely the intramolecular basis set superposition error (BSSE), is discussed. In contrast to out-of-plane vibrations in benzene molecules, diffuse functions on carbon and hydrogen atoms were found to greatly reduce the relative BSSE of n-alkanes. The corrections due to the intramolecular BSSE were found to be almost identical for the MP2, MP4, and CCSD(T) levels of theory. Their cancelation is expected when CCSD(T)/CBS (CBS, complete basis set) energies are evaluated by addition schemes. For larger normal alkanes (N?>?12), the magnitude of the BSSE correction was found to be up to three times larger than the relative stability of the conformer; in this case, the basis set superposition error led to a two orders of magnitude difference in conformer abundance. No error cancelation due to the basis set superposition was found. A comparison with amino acid, peptide, and protein data was provided.     

A comparative coupled cluster and density functional theory study of the HOCI → HCIO transition state

The structural features of the HOCl → HClO isomerization mechanism, including all stationary points, and one saddle point, were examined by use of coupled cluster and the B3LYP density functional theory methodology. To improve the results a very large 6–311++G (3df, 3pd) Gaussian-type basis set was employed in the presented calculations. In addition, Gaussian-3 theory was tested against our coupled cluster (with single, double and triple excitations) results, and they were found to correlate closely with one another by around 1–2 kcal mol^?1. The energy change for this isomerization reaction is predicted to be 54.5 kcalmol^?1 and 52.5 kcalmol^?1 with the B3LYP and CCSD (T) methods, respectively, and the activation barrier is 76.1 kcal mol^?1 and 70.1 kcalmol^?1 with the same methods.     

Quantum chemical studies of the solvation of the hydroxide ion

Abstract

To understand and model the solvation of the hydroxide ion, OH(H₂O)^? _n clusters, n = 1?5, are studied using ab initio quantum chemical techniques, largely at the MP2 level of theory using a double zeta plus polarization functions basis extended by diffuse functions. Energies and vibrational frequencies, together with thermodynamic quantities such as enthalpies, entropies and Gibbs free energies, are computed. This permits comparison with experimental estimates of the successive thermodynamic changes associated with the reaction OH(H₂O)^? _n + H₂O → OH(H₂O)^? _n+1. The theoretical values are in good agreement with experiment. The free energy of hydration of OH^? is modelled by a composite discrete-continuum method where the effects of the first hydration shell (n = 3) are obtained from the gas phase cluster calculation, while the long-range effects are modelled using self consistent reaction field theory, namely by calculating the solvation energy of OH(H₂O)^? _n in a dielectric continuum. The best estimate of the solvation (free) energy at 298 K is ?84·5 kcal mol^?1, compared to the experimental value of ?102·8 kcal mol^?1.     

Further benchmarks of a composite,convergent, statistically calibrated coupled-cluster-based approach for thermochemical and spectroscopic studies

A flexible, high-level, composite approach based on coupled cluster theory has been used to predict the atomization energies and equilibrium structures of 13 small, first-row compounds. Each of the major components in this approach can be systematically improved, thereby providing a practical measure of the inherent uncertainty (or degree of convergence) in the final results. Comparison with Active Thermochemical Table data, which relies on a network of experimental and theoretical data, showed excellent agreement for the atomization energies. With the addition of the latest molecular systems to the Computational Results Database, the composite approach was found to yield a mean absolute deviation of 0.19?kcal?mol^?1 and a root-mean-square deviation of 0.31?kcal?mol^?1 across 142 comparisons. If the analysis is limited to experimental data with estimated uncertainties of 0.2?kcal?mol^?1 or less, the error metrics are cut in half. Similar good agreement is found with experimental structures, but the relative scarcity of accurate equilibrium structures limits the significance of the statistical analysis. Unavoidably, many of the comparisons could not be made with r _e structural parameters. Explicitly correlated methods were found to be effective at replicating results obtained from the standard method with large basis sets, thereby reducing the high computational cost for several of the components.     

Accurate ab initio anharmonic force field and heat of formation for silane

From large basis set coupled cluster calculations and a minor empirical adjustment, an anharmonic force field for silane has been derived that is consistently of spectroscopic quality (±1 cm^?1 on vibrational fundamentals) for all isotopomers of silane studied. Inner-shell polarization functions have an appreciable effect on computed properties and even on anharmonic corrections. From large basis set coupled cluster calculations and extrapolations to the infinite-basis set limit, we obtain TAE₀ = 303.80 ± 0.18 kcal mol^?1, which includes an anharmonic zero-point energy (19.59 kcal mol^?1), inner-shell correlation (—0.36 kcal mol^?1), scalar relativistic corrections (— 0.70 kcal mol^?1) and atomic spin-orbit corrections (—0.43 kcal mol^?1). In combination with the recently revised ΔH ^o _{f, o}[Si(g)], we obtain ΔH ^o _f.o[SiH₄(g)] = 9.9 ± 0.4 kcal mol^?1 in between the two established experimental values.     

Solvent effect on the heat of solution and partial molar volume of magnesium perchlorate

For magnesium perchlorate (MP), the solvent effect on the heat of solution and partial molar volumes (PMV) at 25 °C was studied. Since the complete dissociation of magnesium perchlorate is more difficult to achieve as compared with lithium perchlorate (LP), the concentration dependence of the values of the heat of solution and partial molar volume were noted. Only in highly polar solvents with large donor numbers (DN), such as water, dimethyl sulfoxide, N,N‐dimethyl formamide, and formamide, the differential and integral values of the enthalpies of solution were the same in the range of concentrations studied. In all solvents studied, the values of the heat of solution of MP were highly exothermic and exceed that of LP by more than 30 kcal mol^?1. The values of the partial molar volume of MP were changed from 82.3 cm³ mol^?1 in formamide to ?2.4 cm³ mol^?1 in acetone, and correlate linearly with that of LP (R = 0.975). Taking into account the significant change in the properties of molecules in the solvate shell of cation Mg²⁺, the large increase in the reactivity of reactants, activated by such interaction with Mg²⁺ cation is expected. Copyright © 2010 John Wiley & Sons, Ltd.     

Chiral discrimination in hydrogen-bonded complexes of 2-fluorooxirane with hydrogen peroxide

The paper reports on an ab initio investigation of an extensive series of propylene oxide (PO)···hydrogen peroxide (HOOH) complexes to investigate chiral discrimination. Second-order Møller–Plesset perturbation theory (MP2) with the 6-311++G(d,p) basis set was used. Four complexes of 2–fluorooxirane (FO)···HOOH were identified and their structures as well as their calculated stability ordering were determined. Only–O–is the main interaction point and the–F is not a hydrogen bond acceptor in the four complexes. In this study, the four complexes were defined in a similar way as for PO···HOOH. The largest chirodiastaltic energy between RP-syn and RM-anti is 0.899 kcal mol^?1 and the largest diastereofacial energy between RP-syn and RP-anti is 1.116 kcal mol^?1. For the chiral 2,3-difluorooxirane (R,R), the chirodiastaltic energy is identical to the diastereomeric energy at 0.277 kcal mol^?1. The results obtained were compared with previously reported results on propylene oxide(PO)···HOOH and 2-methylol oxirane(M-olOx)···HOOH complexes. The mechanisms of chiral discrimination in FO···HOOH, PO···HOOH and M-olOx···HOOH were discussed. The harmonic frequencies, IR intensities, rotational constants and dipole moments for the complexes were also presented to assist future spectroscopic investigation.     

Anharmonic effect of the dissociation rate constant of the unimolecular reactions of CH2XCHFO (X = H,F)

This study examines six unimolecular reactions of CH₂XCHFO (X?=?H,?F). The geometries of the reactions are optimized with Gaussian 03. The calculated barrier heights show that bond C–C′ scission, CH₂XCHFO (X?=?H,?F)?→?CH₂X?+?CHFO (R1), dominates the decomposition of CH₂XCHFO. For X?=?H and X?=?F, the barrier heights of (R1) are 13.37 and 9.67?kcal?mol^?1, respectively. The YL (Yao and Lin) method is used to calculate the anharmonic and harmonic rate constants of the unimolecular reactions. The results clearly demonstrate the anharmonic effect of these reactions. In the microcanonical case, for (R1) (X?=?H), the total energy is from 42.78 to 144.84?kcal?mol^?1. The corresponding anharmonic rate constants are from 1.57?×?10¹² to 2.52?×?10^13?s^?1 and the harmonic rate constants are from 1.52?×?10¹² to 2.52?×?10^13?s^?1.     

Computational estimates of the gas-phase basicities,proton affinities and ionization potentials of the six isomers of dihydroxybenzoic acid

The gas-phase basicities (GBs), gas-phase proton affinities (PAs) and ionization potentials (IPs) of all six isomers of dihydroxybenzoic acid have been calculated using density functional theory at the B3LYP/6-311++G(2df,p)//B3LYP/6-31+G** level. A detailed conformational analysis of each isomer was performed, and the calculated thermodynamic properties were Boltzmann averaged over all conformations. Respectively, the GBs and the gas-phase PAs vary from 803.8 and 832.5?kJ?mol^?1 for the least basic species (3,5-DHB) to 830.1 and 861.4?kJ?mol^?1 for the most basic isomer (2,4-DHB). The reported GBs and gas-phase PAs of 2,3-DHB and 2,4-DHB, are in excellent agreement with previous experimental measurements. Agreement for the 2,5-DHB and 3,4-DHB isomers are not as good, but still close to or within the experimental error estimates. The calculated values for the GB and gas-phase PA of 2,6-DHB and especially 3,5-DHB are significantly outside the experimental error brackets. Repeating these calculations on the lowest energy conformation of each isomer at the MP2/6-311++G(2df,p)//MP2/6-31+G** level yielded significantly worse results. Our results indicate that protonation in all isomers takes place on the carboxylic sites. The vertical IPs vary from 8.14 eV for 2,5-DHB to 8.56 eV for 2,4-DHB.     

Computational study on structures,thermochemical properties,and bond energies of disulfide oxygen (S–S–O)‐bridged CH3SSOH and CH3SS(=O)H and radicals

Cleavage of disulfide bonds is a common method used in linking peptides to proteins in biochemical reactions. The structures, internal rotor potentials, bond energies, and thermochemical properties (Δ_fH°, S°, and Cp(T)) of the S–S bridge molecules CH₃SSOH and CH₃SS(=O)H and the radicals CH₃SS?=O and C?H₂SSOH that correspond to H‐atom loss are determined by computational chemistry. Structure and thermochemical parameters (S° and Cp(T)) are determined using density functional Becke, three‐parameter, Lee–Yang–Parr (B3LYP)/6‐31++G (d, p), B3LYP/6‐311++G (3df, 2p). The enthalpies of formation for stable species are calculated using the total energies at B3LYP/6‐31++G (d, p), B3LYP/6‐311++G (3df, 2p), and the higher level composite CBS–QB3 levels with work reactions that are close to isodesmic in most cases. The enthalpies of formation for CH₃SSOH, CH₃SS(=O)H are ?38.3 and ?16.6 kcal mol^?1, respectively, where the difference is in enthalpy RSO–H versus RS(=O)–H bonding. The C–H bond energy of CH₃SSOH is 99.2 kcal mol^?1, and the O–H bond energy is weaker at 76.9 kcal mol^?1. Cleavage of the weak O–H bond in CH₃SSOH results in an electron rearrangement upon loss of the CH₃SSO–H hydrogen atom; the radical rearranges to form the more stable CH₃SS· = O radical structure. Cleavage of the C–H bond in CH₃SS(=O)H results in an unstable [CH₂SS(=O)H]* intermediate, which decomposes exothermically to lower energy CH₂ = S + HSO. The CH₃SS(=O)–H bond energy is quite weak at 54.8 kcal mol^?1 with the H–C bond estimated at between 91 and 98 kcal mol^?1. Disulfide bond energies for CH₃S–SOH and CH3S–S(=O)H are low: 67.1 and 39.2 kcal mol^?1. Copyright © 2011 John Wiley & Sons, Ltd.     

Adsorption of mercaptopurine drug on the BN nanotube,nanosheet and nanocluster: a density functional theory study

ABSTRACT

We have investigated the interaction of mercaptopurine (MP) drug with BN nanotube, nanosheet and nanocluster using density functional theory calculations in the gas phase, and aqueous solution. We predicted that the MP drug tends to be physically adsorbed on the surface of BN nanosheet with an adsorption energy (E_ad) about ?3.2?kcal/mol. The electronic properties of BN nanosheet are not affected by the MP drug, and this sheet is not a sensor. But the electronic properties of BN nanotube and nanocluster are significantly sensitive to this drug in both gas phase, and aqueous solution. The BN nanocluster suffers from a long recovery time (8.8?×?10⁸?s) because of a strong interaction (E_ad?=??28.6?kcal/mol), and this cluster is not a proper sensor for MP detection. But the BN nanotube benefits from a short recovery time about 49.5?s at room temperature, and may be a promising candidate for application in the MP sensors. The water solvent decreases the strength of interaction between the BN nanotube, and MP drug, but it does not affect the electronic sensitivity of the nanotube sensibly.     

Combined Raman scattering and ab initio investigation of the interaction between pyrene and carbon SWNT

Raman spectra of HiPco SWNT and SWNT-pyrene films were measured in the 160–1800 cm^?1 range. Due to the non-covalent interaction between SWNT and pyrene the most intensive component of the SWNT G mode (1590 cm^?1) is downshifted by 2 cm^?1 and becomes narrower. Also the intensity of the low-frequency component of the G mode (1550 cm^?1) decreases by about 30%. Structures and interaction energies in the complexes of pyrene and zigzag (n, 0) SWNTs [6 ≤ n ≤ 20] were determined at the MP2 level of theory. The BSSE-free geometry optimization of the pyrene-zigzag (12,0) SWNT complex converged to a structure with a 1/2staggered conformation and with an intermolecular distance of 3.5 Å. The BSSE-free interaction energy in the complex is ?30.8 kj mol^?1. Increasing of the nanotube diameter leads to a higher interaction energy. This energy becomes equal to ?37.2 kJ mol^?1 in the case of a planar carbon surface.