Accurate CCSD Research Articles

Further analysis and comparative study of intermolecular interactions using dimers from the S22 database.

Accurate MP2 and CCSD(T) complete basis set (CBS) interaction energy curves (14 points for each curve) have been obtained for 20 of the dimers reported in the S22 set and analytical Morse curves have been fitted that can be used in developing updated density functional theory (DFT) and force field models. The magnitude and the effect of the basis set superposition error (BSSE) were carefully investigated. We found that going up to aug-cc-pVDZ and aug-cc-pVTZ basis sets is enough to obtain accurate CBS MP2 energies when BSSE corrected values are used but aug-cc-pVTZ and aug-cc-pVQZ basis sets are needed when the BSSE uncorrected total energies are used in CBS extrapolations. MP2 interaction energies with smaller basis sets such as 6-31G* are found to have very little dispersion energy and that the true source of dispersion attributed attractive interactions is almost entirely due to BSSE. MP2 and CCSD(T) CBS interaction energies are found to be very close to one another if aromatic systems are not involved. Comparative analyses have been performed with semiempirical and ab initio methods utilizing the moderate in size but affordable 6-31G* basis set both of which can be readily applied to macromolecular systems. The new M06-2X and M06-L DFT functionals were found to be more accurate than all methods tested herein. Interaction energy curves using the SG1 grid showed discontinuities for several of the dimer systems but this problem disappeared when finer DFT numerical grids were used.

On the Nature of Bonding in Lone Pair···π-Electron Complexes: CCSD(T)/Complete Basis Set Limit Calculations.

The nature of the stabilization in lone pair···π-electron complexes was investigated using the highly accurate CCSD(T) method based on the complete basis set limit, as well as the DFT-SAPT perturabative method. Specifically, we studied various structures of benzene···water, benzene···dimethylether, and 1,2,4,5-tetracyanobenzene···water complexes. The lone pair···π-electron interactions between an unsubstituted aromatic ring and a water molecule are repulsive in the whole range of vertical distances. Partial stabilization results by rotating the water molecule by 90° (with the water and aromatic ring being localized in parallel planes) or by decreasing the negative charge at oxygen and simultaneously increasing the polarizability of the system, which provides stabilization even for genuine lone pair···π-electron interactions. In these cases, a substantial part of the stabilization stems from dispersion energy. Substituting an aromatic ring by electron-withdrawing cyano groups represents the most powerful way to achieve a substantial stabilization of genuine lone pair···π-electron interactions. This stabilization is comparable to quite strong H-bonding, originating in electrostatic and, to a slightly lesser degree, dispersion energies.

Enthalpy of formation and anharmonic force field of diacetylene

The enthalpy of formation of diacetylene (C4H2) is pinpointed using state-of-the-art theoretical methods, accounting for high-order electron correlation, relativistic effects, non-Born-Oppenheimer corrections, and vibrational anharmonicity. Molecular energies are determined from coupled cluster theory with single and double excitations (CCSD), perturbative triples [CCSD(T)], full triples (CCSDT), and perturbative quadruples [CCSDT(Q)], in concert with correlation-consistent basis sets (cc-pVXZ, X=D, T, Q, 5, 6) that facilitate extrapolations to the complete basis set limit. The first full quartic force field of diacetylene is determined at the highly accurate all-electron CCSD(T) level with a cc-pCVQZ basis, which includes tight functions for core correlation. Application of second-order vibrational perturbation theory to our anharmonic force field yields fundamental frequencies with a mean absolute difference of only 3.9 cm(-1) relative to the experimental band origins, without the use of any empirical scale factors. By a focal point approach, we converge on an enthalpy change for the isogyric reaction 2 H-C[triple bond]C-H-->H-C[triple bond]C-C[triple bond]C-H+H2 of (+0.03, +0.81) kcal mol(-1) at (0, 298.15) K. With the precisely established fHdegrees of acetylene, we thus obtain DeltafHdegrees(C4H2)=(109.4,109.7)+/-0.3 kcal mol(-1) at (0, 298.15) K. Previous estimates of the diacetylene enthalpy of formation range from 102 to 120 kcal mol(-1).

Scaled MP3 Non‐Covalent Interaction Energies Agree Closely with Accurate CCSD(T) Benchmark Data

Scaled MP3 interaction energies calculated as a sum of MP2/CBS (complete basis set limit) interaction energies and scaled third-order energy contributions obtained in small or medium size basis sets agree very closely with the estimated CCSD(T)/CBS interaction energies for the 22 H-bonded, dispersion-controlled and mixed non-covalent complexes from the S22 data set. Performance of this so-called MP2.5 (third-order scaling factor of 0.5) method has also been tested for 33 nucleic acid base pairs and two stacked conformers of porphine dimer. In all the test cases, performance of the MP2.5 method was shown to be superior to the scaled spin-component MP2 based methods, e.g. SCS-MP2, SCSN-MP2 and SCS(MI)-MP2. In particular, a very balanced treatment of hydrogen-bonded compared to stacked complexes is achieved with MP2.5. The main advantage of the approach is that it employs only a single empirical parameter and is thus biased by two rigorously defined, asymptotically correct ab-initio methods, MP2 and MP3. The method is proposed as an accurate but computationally feasible alternative to CCSD(T) for the computation of the properties of various kinds of non-covalently bound systems.

Heats of Formation and Bond Energies of the H(3−n)BXn Compounds for (X = F, Cl, Br, I, NH2, OH, and SH)

Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for the borane compounds H(3-n)BX(n) for (X = F, Cl, Br, I, NH2, OH, and SH) and various radicals from coupled cluster theory (CCSD(T)) calculations with an effective core potential correlation-consistent basis set for I. In order to achieve near chemical accuracy (+/-1.5 kcal/mol), three corrections were added to the complete basis set binding energies calculated from frozen core coupled cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, and a correction for first-order atomic spin-orbit effects. Vibrational zero point energies were computed at the MP2 level. The calculated heats of formation are in excellent agreement with the available experimental data for the closed shell molecules, but show larger differences with the reported "experimental" values for the BX2 radicals. The heats of formation of the BX2 radicals were also calculated at the G3(MP2) level of theory, and the values were in excellent agreement with the more accurate CCSD(T) values. On the basis of extensive comparisons with experiment for a wide range of compounds, our calculated values for these radicals should be good to +/-1.5 kcal/mol and thus are to be preferred over the experimental values. The accurately calculated heats of formation allow us to predict the B-X and B-H adiabatic bond dissociation energies (BDEs) to within +/-1.5 kcal/mol. The B-F BDEs in the H(3-n)BF(n) compounds and in BF (1Sigma+) are the largest BDEs in comparison to the other substituents that were investigated. The second and third largest B-X BDEs in the H(3-n)BX(n) and BX compounds are predicted for X = OH and NH2, respectively. The substituents have a minimal effect on the B-H BDEs in HBX2 and H2BX compared to the first B-H BDE of borane. The differences in adiabatic and diabatic BDEs, which are related to the reorganization energy in the product, can be estimated from singlet-triplet splittings in these molecules, and can account for the large fluctuations in adiabatic BDEs observed, specifically for the BX2 and HBX radicals, during the stepwise loss of the respective substituents.

Benzene Dimer: High-Level Wave Function and Density Functional Theory Calculations.

High-level OVOS (optimized virtual orbital space) CCSD(T) interaction energy calculations (up to the aug-cc-pVQZ basis set) and various extrapolations toward the complete basis set (CBS) limit are presented for the most important structures on the benzene dimer potential energy surface. The geometries of these structures were obtained via an all-coordinate gradient geometry optimization using the DFT-D/BLYP method, covering the empirical dispersion correction fitted exclusively for this system. The fit was carried out against two estimated CCSD(T)/CBS potential energy curves corresponding to the distance variation between two benzene rings for the parallel-displaced (PD) and T-shaped (T) structures. The effect of the connected quadruple excitations on the interaction energy was estimated using the CCSD(TQf) method in a 6-31G*(0.25) basis set, destabilizing the T and T-shaped tilted (TT) structures by ≈0.02 kcal/mol and the PD structure by ≈0.04 kcal/mol. Our best CCSD(T)/CBS results show, within the error bars of the applied methodology, that the energetically lowest-lying structure is the TT structure, which is nearly 0.1 kcal/mol more stable than the almost isoenergetic PD and T structures. The specifically parametrized DFT-D/BLYP method leads to a correct energy ordering of the structures, with the errors being smaller by 0.2 kcal/mol with respect to the most accurate CCSD(T) values.

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 DeltaCCSD(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.

The polarizabilities of small stoichiometric aluminum phosphide clusters Al nP n ( n = 2–9). Ab initio and density functional investigation

The mean dipole polarizabilities and the polarizability anisotropies of stoichiometric Al n P n ( n = 2–9) clusters are investigated and presented for the first time. Basis set augmentation effects on the 6-31+G substrate are studied and discussed. The electron correlation contributions to the polarizability were studied for clusters up to 12 atoms at MP2 and CCSD(T) levels of theory, and the performance of the widely used B3LYP, B3PW91, mPW1PW91 functionals are assessed. From the methodological standpoint, our results show that for clusters where the ionic contribution to the bonding is strong the MP2 method yields polarizability values in very good agreement with the more accurate CCSD(T) approximation.

Production of HNC from the CH(X 2Π) + NH(X 3Σ −) reaction: Direct dynamics study

The production mechanism and dynamics of the HNC molecule from the radical–radical CH + NH reaction on the lowest doublet potential energy surface has been theoretically studied using electronic structure calculations. We have applied a direct dynamics simulation technique at the B3LYP/6-311++G(d,p) level of theory. The accuracy of the B3LYP-level potential energy surface was systematically confirmed from the comparison to more accurate MRCI-level and CCSD(T)-level calculations. More than 250 classical trajectories were integrated and we have found that the CH + NH reaction somewhat favors the HNC + H production channel over the HCN + H production despite that the energy level of HCN + H is lower than that of HNC + H. Detailed reaction mechanisms will be discussed on the basis of the trajectory results.

Structural Investigation of Asymmetrical Dimer Radical Cation System (H2O−H2S)+: Proton-Transferred or Hemi-Bonded?

Ab initio molecular orbital and hybrid density functional methods have been employed to characterize the structure and bonding of (H2O-H2S)+, an asymmetrical dimer radical cation system. A comparison has been made between the two-center three-electron (2c-3e) hemi-bonded system and the proton-transferred hydrogen-bonded systems of (H2O-H2S)+. Geometry optimization of these systems was carried out using unrestricted Hartree Fock (HF), density functional theory with different functionals, and second-order Møller-Plesset perturbation (MP2) methods with 6-311++G(d,p) basis set. Hessian calculations have been done at the same level to check the nature of the equilibrium geometry. Energy data were further improved by calculating basis set superposition error for the structures optimized through MP2/6-311++G(d,p) calculations. The calculated results show that the dimer radical cation structure with H2O as proton acceptor is more stable than those structures in which H2O acts as a proton donor or the 2c-3e hemi-bonded (H2O thereforeSH2)+ system. This stability trend has been further confirmed by more accurate G3, G3B3, and CCSD(T) methods. On the basis of the present calculated results, the structure of H4OS+ can best be described as a hydrogen-bonded complex of H3O+ and SH with H2O as a proton acceptor. It is in contrast to the structure of neutral (H2O...H2S) dimer where H2O acts as a proton donor. The present work has been able to resolve the ambiguity in the nature of bonding between H2O and H2S in (H2O-H2S)+ asymmetrical dimer radical cation.

Aliphatic C−H/π Interactions: Methane−Benzene, Methane−Phenol, and Methane−Indole Complexes

Noncovalent C-H/pi interactions are prevalent in biochemistry and are important in molecular recognition. In this work, we present potential energy curves for methane-benzene, methane-phenol, and methane-indole complexes as prototypes for interactions between C-H bonds and the aromatic components of phenylalanine, tyrosine, and tryptophan. Second-order perturbation theory (MP2) is used in conjunction with the aug-cc-pVDZ and aug-cc-pVTZ basis sets to determine the counterpoise-corrected interaction energy for selected complex configurations. Using corrections for higher-order electron correlation determined with coupled-cluster theory through perturbative triples [CCSD(T)] in the aug-cc-pVDZ basis set, we estimate, through an additive approximation, results at the very accurate CCSD(T)/aug-cc-pVTZ level of theory. Symmetry-adapted perturbation theory (SAPT) is employed to determine the physically significant components of the total interaction energy for each complex.

Vibrational and electron paramagnetic resonance properties of free and MgO supported AuCO complexes

The bonding, spin density related properties, and vibrational frequency of CO bound to single Au atom in the gas-phase or supported on MgO surfaces have been investigated with a variety of computational methods and models: periodic plane waves calculations have been compared with molecular approaches based on atomic orbital basis sets; pseudopotential methods with all electron fully relativistic calculations; various density functional theory (DFT) exchange-correlation functionals with the unrestricted coupled-cluster singles and doubles with perturbative connected triples [CCSD(T)]. AuCO is a bent molecule but the potential for bending is very soft, and small changes in the bond angle result in large changes in the CO gas-phase vibrational frequency. At the equilibrium geometry the DFT calculated vibrational shift of CO with respect to the free molecule is about -150 cm(-1), whereas smaller values -60-70 cm(-1) are predicted by the more accurate CCSD(T) method. These relatively large differences are due to the weak and nonclassic bonding in this complex. Upon adsorption on MgO, the CO vibrational shift becomes much larger, about -290 cm(-1), due to charge transfer from the basic surface oxide anion to AuCO. This large redshift is predicted by all methods, and is fully consistent with that measured for MgOAuCO complexes. The strong influence of the support on the AuCO bonding is equally well described by all different approaches.

Static Dipole Polarizability and Hyperpolarizability of F2 from Density Functional Theory Calculations. Similarities and Dissimilarities with Conventional ab initio Results

We have calculated the bond-length dependence of the static dipole polarizability and hyperpolarizability of F2 relying on finite-field density functional calculations. At the internuclear separation of Re = 2.66816 a0 we obtain α/ e2a0 2Eh-1 = 8.73 (B3LYP) and 8.60 (B3PW91),α./ e2a0 2Eh-1 = 6.42 (B3LYP) and 6.32 (B3PW91), γ / e4a04Eh-3 = 535 (B3LYP) and 503 (B3PW91) with a large, flexible [9s6p4d1f] basis set. For all three properties we observe that both B3LYPand B3PW91 yield values above the more accurate CCSD(T) results: CCSD(T) < B3PW91 < B3LYP. The discrepancy between DFT and conventional ab initio methods is more pronounced for the anisotropy of the (hyper)polarizability.

CCSD(T), W1, and other model chemistry predictions for gas‐phase deprotonation reactions

AbstractA series of CCSD(T) single‐point calculations on MP4(SDQ) geometries and the W1 model chemistry method have been used to calculate ΔH° and ΔG° values for the deprotonation of 17 gas‐phase reactions where the experimental values have reported accuracies within 1 kcal/mol. These values have been compared with previous calculations using the G3 and CBS model chemistries and two DFT methods. The most accurate CCSD(T) method uses the aug‐cc‐pVQZ basis set. Extrapolation of the aug‐cc‐pVTZ and aug‐cc‐pVQZ results yields the most accurate agreement with experiment, with a standard deviation of 0.58 kcal/mol for ΔG° and 0.70 kcal/mol for ΔH°. Standard deviations from experiment for ΔG° and ΔH° for the W1 method are 0.95 and 0.83 kcal/mol, respectively. The G3 and CBS‐APNO results are competitive with W1 and are much less expensive. Any of the model chemistry methods or the CCSD(T)/aug‐cc‐pVQZ method can serve as a valuable check on the accuracy of experimental data reported in the National Institutes of Standards and Technology (NIST) database. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Dissolution Nature of Cesium Fluoride by Water Molecules

The structures, stabilities, thermodynamic quantities, dissociation energies, infrared spectra, and electronic properties of CsF hydrated by water molecules are investigated by using density functional theory, Møller-Plesset second-order perturbation theory (MP2), coupled cluster theory with singles, doubles, and perturbative triples excitations (CCSD(T)), and ab initio molecular dynamic (AIMD) simulations. It is revealed that at 0 K three water molecules (as a global minimum structure) begin to half-dissociate the Cs-F, and six water molecules (though not a global minimum energy structure) can dissociate it. By the combination of the accurate CCSD(T) conformational energies for Cs(H2O)6 at 0 K with the AIMD thermal energy contribution, it reveals that the half-dissociated structure is the most stable at 0 K, but this structure (which is still the most stable) changes to the dissociated structure above 50 K. The spectra of CsF(H2O)(1-6) from MP2 calculations and the power spectra of CsF(H2O)6 from 50 and 100 K AIMD simulations are also reported.

Intermolecular Potential of the O2−O2 Dimer. An ab Initio Study and Comparison with Experiment

Accurate intermolecular potentials for the lowest three multiplet states of O2-O2 dimer have been produced on the basis of ab initio calculations. The quintet potential was taken from previous highly correlated CCSD(T) calculations. In this work, we perform MRCI calculations, with large basis sets including bond functions, of the singlet and triplet states, which are of multireference character. As expected the size inconsistency and lack of higher order excitations limit the accuracy of the MRCI potentials specifically in describing the long range interactions. We show that the Heisenberg Hamiltonian provides an accurate representation of the exchange interactions in this system and this enables us to combine the accurate CCSD(T) potentials with the MRCI spin-exchange parameter to obtain accurate singlet and triplet potentials. The reliability of these potentials is tested by computing integral cross sections and comparing them with the detailed experimental study of the Perugia group, with excellent results. More interestingly, comparison with the experimentally derived potential shows important discrepancies for some angular orientations including that corresponding with the global minima, indicating the need for further work, both theoretical and experimental, to clarify their origin.

Validation of Density Functional Methods for Computing Structures and Energies of Mercury(IV) Complexes

AbstractFor Abstract see ChemInform Abstract in Full Text.

The performance of density functional theory for LnF (Ln=Nd, Eu, Gd, Yb) and YbH

Different density functional theory (DFT) functionals have been evaluated by studying geometries and bond strengths of YbH, YbF, EuF, GdF, and NdF and compared with accurate CCSD(T) results and, when available, experiment. The agreement between the CCSD(T) results and experiment, when available, is good. The agreement is also good between bond strengths calculated at the DFT level using relativistic effective core potentials and the CCSD(T) results. However, the all-electron ADF calculations systematically overestimate binding energies. The geometries obtained by both the all-electron and the effective-core-potential-based DFT calculations are generally in good agreement with the CCSD(T) results.

Ab initio calculation of the refractivity and hyperpolarizability second virial coefficients of neon gas

Second virial coefficients for the density dependence of a number of electric properties are calculated for neon gas. Employing an accurate CCSD(T) potential for the Ne2 van der Waals dimer and interaction-induced electric dipole polarizabilities and hyperpolarizabilities obtained from CCSD response theory, we evaluated the dielectric, refractivity, Kerr and ESHG second virial coefficients using both a semiclassical and a quantum statistical approach. The results cover a wide range of temperatures and are expected to be more reliable than the available experimental and empirical data. Quantum effects are found to be important only for temperatures below 100 K. The frequency-dependence of the refractivity virial coefficient is found to be small, but not negligible. For frequencies in the visible region it accounts for a few percent of the final results. For the ESHG virial coefficient of neon, frequency dependence is found to be very important, accounting for 20–25% of the second virial coefficient at the typical frequencies employed in experiments.

Electric Quadrupole and Hexadecapole Moment, Dipole and Quadrupole Polarizability, Second Electric Dipole Hyperpolarizability for P2, and a Comparative Study of Molecular Polarization in N2, P2, and As2

We have obtained electric properties of P⋮P from finite-field Møller−Plesset perturbation theory, density functional theory and coupled cluster techniques. Reference, near-Hartree−Fock values have been obtained with a very large (20s15p10d5f) uncontracted basis set consisting of 300 Gaussian-type functions. At the experimental equilibrium bond length of Re = 1.8934 Å we obtain self-consistent field values of 1.0682 ea02 for the quadrupole moment (ϑ), −41.68 ea04 for the hexadecapole moment (Φ), 51.16 for the mean ( ) and 28.58 e2a02Eh-1 for the anisotropy of the dipole polarizability, and 16.5 × 103 e4a04Eh-3 for the mean second dipole hyperpolarizability ( ). Electron correlation reduces strongly the magnitude of the electric moments. Both components of the dipole polarizability are reduced by electron correlation, but a small increase is observed for the dipole hyperpolarizability. Our best post-Hartree−Fock values have been obtained with a [9s7p5d3f] basis set at the CCSD(T) level of theory: ϑ = 0.4850 ea02, Φ = −31.25 ea04, = 49.20 and Δα = 28.02 e2a02Eh-1, = 16.8 × 103 e4a04Eh-3. The bond-length dependence around Re has been obtained for all properties. Conventional density functional theory methods predict dipole polarizabilities close enough to the most accurate CCSD(T) values but overestimate the second dipole hyperpolarizability. The mean dipole polarizability changes as (NaK) > (AlCl) > (SiS) > (P2) > (Zn) for some isoelectronic, 30-electron systems. It is seen that the electric properties in the sequence N2→P2→As2 display regular changes.