MP2 Binding Energies Research Articles

Monte Carlo MP2-F12 for Noncovalent Interactions: The C60 Dimer.

A scalable stochastic algorithm is presented that can evaluate explicitly correlated (F12) second-order many-body perturbation (MP2) energies of weak, noncovalent, intermolecular interactions. It first transforms the formulas of the MP2 and F12 energy differences into a short sum of high-dimensional integrals of Green's functions in real space and imaginary time. These integrals are then evaluated by the Monte Carlo method augmented by parallel execution, redundant-walker convergence acceleration, direct-sampling autocorrelation elimination, and control-variate error reduction. By sharing electron-pair walkers across the supermolecule and its subsystems spanned by the joint basis set, the statistical uncertainty is reduced by one to 2 orders of magnitude in the MP2 binding energy corrected for the basis-set incompleteness and superposition errors. The method predicts the MP2-F12/aug-cc-pVDZ binding energy of 19.1 ± 4.0 kcal mol-1 for the C60 dimer at the center distance of 9.748 Å.

Complexes H2 CO:PXH2 and HCO2 H : PXH2 for X=NC, F, Cl, CN, OH, CCH, CH3 , and H: Pnicogen Bonds and Hydrogen Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate H2 CO : PXH2 pnicogen-bonded complexes and HCO2 H : PXH2 complexes that are stabilized by pnicogen bonds and hydrogen bonds, with X=NC, F, Cl, CN, OH, CCH, CH3 , and H. The binding energies of these complexes exhibit a second-order dependence on the O-P distance. DFT-SAPT binding energies correlate linearly with MP2 binding energies. The HCO2 H : PXH2 complexes are stabilized by both a pnicogen bond and a hydrogen bond, resulting in greater binding energies for the HCO2 H : PXH2 complexes compared to H2 CO : PXH2 . Neither the O-P distance across the pnicogen bond nor the O-P distance across the hydrogen bond correlates with the binding energies of these complexes. The nonlinearity of the hydrogen bonds suggests that they are relatively weak bonds, except for complexes in which the substituent X is either CH3 or H. The pnicogen bond is the more important stabilizing interaction in the HCO2 H : PXH2 complexes except when the substituent X is a more electropositive group. EOM-CCSD spin-spin coupling constants 1p J(O-P) across pnicogen bonds in H2 CO:PXH2 and HCO2 H : PXH2 complexes increase as the O-P distance decreases, and exhibit a second order dependence on that distance. There is no correlation between 2h J(O-P) and the O-P distance across the hydrogen bond in the HCO2 H : PXH2 complexes. 2h J(O-P) coupling constants for complexes with X=CH3 and H have much greater absolute values than anticipated from their O-P distances.

Divergence of Many-Body Perturbation Theory for Noncovalent Interactions of Large Molecules.

Prompted by recent reports of large errors in noncovalent interaction (NI) energies obtained from many-body perturbation theory (MBPT), we compare the performance of second-order Mo̷ller-Plesset MBPT (MP2), spin-scaled MP2, dispersion-corrected semilocal density functional approximations (DFAs), and post-Kohn-Sham random phase approximation (RPA) for predicting binding energies of supramolecular complexes contained in the S66, L7, and S30L benchmarks. All binding energies are extrapolated to the basis set limit, corrected for basis set superposition errors, and compared to reference results of the domain-based local pair-natural orbital coupled-cluster (DLPNO-CCSD(T)) or better quality. Our results confirm that MP2 severely overestimates binding energies of large complexes, producing relative errors of over 100% for several benchmark compounds. RPA relative errors consistently range between 5 and 10%, significantly less than reported previously using smaller basis sets, whereas spin-scaled MP2 methods show limitations similar to MP2, albeit less pronounced, and empirically dispersion-corrected DFAs perform almost as well as RPA. Regression analysis reveals a systematic increase of relative MP2 binding energy errors with the system size at a rate of approximately 0.1% per valence electron, whereas the RPA and dispersion-corrected DFA relative errors are virtually independent of the system size. These observations are corroborated by a comparison of computed rotational constants of organic molecules to gas-phase spectroscopy data contained in the ROT34 benchmark. To analyze these results, an asymptotic adiabatic connection symmetry-adapted perturbation theory (AC-SAPT) is developed, which uses monomers at full coupling, whose ground-state density is constrained to the ground-state density of the complex. Using the fluctuation-dissipation theorem, we obtain a nonperturbative "screened second-order" expression for the dispersion energy in terms of monomer quantities, which is exact for non-overlapping subsystems and free of induction terms; a first-order RPA-like approximation to the Hartree, exchange, and correlation kernel recovers the macroscopic Lifshitz limit. The AC-SAPT expansion of the interaction energy is obtained from Taylor expansion of the coupling strength integrand. Explicit expressions for the convergence radius of the AC-SAPT series are derived within RPA and MBPT and numerically evaluated. While the AC-SAPT expansion is always convergent for nondegenerate monomers when RPA is used, it is found to spuriously diverge for second-order MBPT, except for the smallest and least polarizable monomers. The divergence of the AC-SAPT series for MBPT is numerically confirmed within RPA; prior numerical results on the convergence of the SAPT expansion for MBPT methods are revisited and support this conclusion once sufficiently high orders are included. The cause of the failure of MBPT methods for NIs of large systems is missing or incomplete "electrodynamic" screening of the Coulomb interaction due to induced particle-hole pairs between electrons in different monomers, leaving the effective interaction too strong for AC-SAPT to converge. Hence, MBPT cannot be considered reliable for quantitative predictions of NIs, even in moderately polarizable molecules with a few tens of atoms. The failure to accurately account for electrodynamic polarization makes MBPT qualitatively unsuitable for applications such as NIs of nanostructures, macromolecules, and soft materials; more robust nonperturbative approaches such as RPA or coupled cluster methods should be used instead whenever possible.

Theoretical Description of R-X⋯NH3 Halogen Bond Complexes: Effect of the R Group on the Complex Stability and Sigma-Hole Electron Depletion.

In the present work, a number of R–X⋯NH3 (X = Cl, Br, and I) halogen bonded systems were theoretical studied by means of DFT calculations performed at the ωB97XD/6-31+G(d,p) level of theory in order to get insights on the effect of the electron-donating or electron-withdrawing character of the different R substituent groups (R = halogen, methyl, partially fluorinated methyl, perfluoro-methyl, ethyl, vinyl, and acetyl) on the stability of the halogen bond. The results indicate that the relative stability of the halogen bond follows the Cl < Br < I trend considering the same R substituent whereas the more electron-withdrawing character of the R substituent the more stable the halogen bond. Refinement of the latter results, performed at the MP2/6-31+G(d,p) level showed that the DFT and the MP2 binding energies correlate remarkably well, suggesting that the Grimme’s type dispersion-corrected functional produces reasonable structural and energetic features of halogen bond systems. DFT results were also observed to agree with more refined calculations performed at the CCSD(T) level. In a further stage, a more thorough analysis of the R–Br⋯NH3 complexes was performed by means of a novel electron localization/delocalization tool, defined in terms of an Information Theory, IT, based quantity obtained from the conditional pair density. For the latter, our in-house developed C++/CUDA program, called KLD (acronym of Kullback–Leibler divergence), was employed. KLD results mapped onto the one-electron density plotted at a 0.04 a.u. isovalue, showed that (i) as expected, the localized electron depletion of the Br sigma-hole is largely affected by the electron-withdrawing character of the R substituent group and (ii) the R–X bond is significantly polarized due to the presence of the NH3 molecule in the complexes. The afore-mentioned constitutes a clear indication of the dominant character of electrostatics on the stabilization of halogen bonds in agreement with a number of studies reported in the main literature. Finally, the cooperative effects on the [Br—CN]n system (n = 1–8) was evaluated at the MP2/6-31+G(d,p) level, where it was observed that an increase of about ~14.2% on the complex stability is obtained when going from n = 2 to n = 8. The latter results were corroborated by the analysis of the changes on the Fermi-hole localization pattern on the halogen bond zones, which suggests an also important contribution of the electron correlation in the stabilization of these systems.

Breaking the bottleneck: Use of molecular tailoring approach for the estimation of binding energies at MP2/CBS limit for large water clusters.

A pragmatic method based on the molecular tailoring approach (MTA) for estimating the complete basis set (CBS) limit at Møller-Plesset second order perturbation (MP2) theory accurately for large molecular clusters with limited computational resources is developed. It is applied to water clusters, (H2O)n (n = 7, 8, 10, 16, 17, and 25) optimized employing aug-cc-pVDZ (aVDZ) basis-set. Binding energies (BEs) of these clusters are estimated at the MP2/aug-cc-pVNZ (aVNZ) [N = T, Q, and 5 (whenever possible)] levels of theory employing grafted MTA (GMTA) methodology and are found to lie within 0.2 kcal/mol of the corresponding full calculation MP2 BE, wherever available. The results are extrapolated to CBS limit using a three point formula. The GMTA-MP2 calculations are feasible on off-the-shelf hardware and show around 50%-65% saving of computational time. The methodology has a potential for application to molecular clusters containing ∼100 atoms.

An energy decomposition analysis for second-order Møller-Plesset perturbation theory based on absolutely localized molecular orbitals.

An energy decomposition analysis (EDA) of intermolecular interactions is proposed for second-order Møller-Plesset perturbation theory (MP2) based on absolutely localized molecular orbitals (ALMOs), as an extension to a previous ALMO-based EDA for self-consistent field methods. It decomposes the canonical MP2 binding energy by dividing the double excitations that contribute to the MP2 wave function into classes based on how the excitations involve different molecules. The MP2 contribution to the binding energy is decomposed into four components: frozen interaction, polarization, charge transfer, and dispersion. Charge transfer is defined by excitations that change the number of electrons on a molecule, dispersion by intermolecular excitations that do not transfer charge, and polarization and frozen interactions by intra-molecular excitations. The final two are separated by evaluations of the frozen, isolated wave functions in the presence of the other molecules, with adjustments for orbital response. Unlike previous EDAs for electron correlation methods, this one includes components for the electrostatics, which is vital as adjustment to the electrostatic behavior of the system is in some cases the dominant effect of the treatment of electron correlation. The proposed EDA is then applied to a variety of different systems to demonstrate that all proposed components behave correctly. This includes systems with one molecule and an external electric perturbation to test the separation between polarization and frozen interactions and various bimolecular systems in the equilibrium range and beyond to test the rest of the EDA. We find that it performs well on these tests. We then apply the EDA to a halogen bonded system to investigate the nature of the halogen bond.

Importance and Reliability of Small Basis Set CCSD(T) Corrections to MP2 Binding and Relative Energies of Water Clusters

MP2 describes hydrogen-bonded systems well, yet a higher-order electron correlation correction in the form of a CCSD(T) calculation is usually necessary to achieve benchmark quality energies. We evaluated the importance and reliability of small basis set CCSD(T) corrections to MP2 (δ(MP2)(CCSD(T))) both on the binding (ΔE) and relative (ΔΔE) energies for a large number of systems including four water dimer stationary points and 57 other clusters up to undecamers, (H2O)11. By comparing the MP2 energies with CCSD(T) and the explicitly correlated MP2-F12 energies with variants of CCSD(T)-F12 using different basis sets, we were able to establish that the correction to the binding energy (ΔE) is sensitive to basis set size, especially for small double-ζ basis sets. On the other hand, the basis set sensitivity of the correction to the relative energy (ΔΔE) within each cluster size is very small. While the δ(MP2)(CCSD(T)) correction to the binding energy might vary in magnitude with basis set size, its effect on relative energy (and hence the stability of isomers) is remarkably consistent. Therefore, we recommend the inclusion of this correction to obtain the relative stability of closely spaced isomers using a double-ζ basis set with polarization and diffuse functions such as aug-cc-pVDZ.

Homogeneous and heterogeneous noncovalent dimers of formaldehyde and thioformaldehyde: structures, energetics, and vibrational frequencies.

This work provides the first characterization of five stationary points of the homogeneous thioformaldehyde dimer, (CH2S)2, and seven stationary points of the heterogeneous formaldehyde/thioformaldehyde dimer, CH2O/CH2S, with correlated ab initio electronic structure methods. Full geometry optimizations and corresponding harmonic vibrational frequencies were computed with second-order Møller-Plesset perturbation theory (MP2) and 13 different density functionals in conjunction with triple-ζ basis sets augmented with diffuse and multiple sets of polarization functions. The MP2 results indicate that the three stationary points of (CH2S)2 and four of CH2O/CH2S are minima, in contrast to two stationary points of the formaldehyde dimer, (CH2O)2. Single-point energies were also computed using the explicitly correlated MP2-F12 and CCSD(T)-F12 methods and basis sets as large as heavy-aug-cc-pVTZ. The (CH2O)2 and CH2O/CH2S MP2 and MP2-F12 binding energies deviated from the CCSD(T)-F12 binding energies by no more than 0.2 and 0.4 kcal mol(-1), respectively. The (CH2O)2 and CH2O/CH2S global minimum is the same at every level of theory. However, the MP2 methods overbind (CH2S)2 by as much as 1.1 kcal mol(-1), effectively altering the energetic ordering of the thioformaldehyde dimer minima relative to the CCSD(T)-F12 energies. The CCSD(T)-F12 binding energies of the (CH2O)2 and CH2O/CH2S stationary points are quite similar, with the former ranging from around -2.4 to -4.6 kcal mol(-1) and the latter from about -1.1 to -4.4 kcal mol(-1). Corresponding (CH2S)2 stationary points have appreciably smaller CCSD(T)-F12 binding energies ranging from ca. -1.1 to -3.4 kcal mol(-1). The vibrational frequency shifts upon dimerization are also reported for each minimum on the MP2 potential energy surfaces.

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 C2 h 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.

The MP2 binding energy of the ethene dimer and its dependence on the auxiliary basis sets: a benchmark study using a newly developed infrastructure for the processing of quantum chemical data

Data intense processes such as the establishment of quantitative structure–property relationships, the design of novel compounds and also the validation of new quantum chemical methods call for a structured approach for the processing of the results. Whereas in quantum chemistry there are established de facto standards for the computational methods, community standards for [exchange-] data formats are still under development. In this article we present a benchmark study of the (auxiliary-) basis set dependence of the binding energies of an array of conformers of the ethene dimer using the RI-MP2-F12 method. The study was performed using a version of the TURBOMOLE program package modified to provide output in an extended CML format to be imported in an eXist database. This infrastructure for the generation, archival, analysis, and exchange of quantum chemical data is briefly introduced in this article.

Ab Initio Study of Molecular Hydrogen Adsorption in Covalent Organic Framework-1

The adsorption energies of hydrogen molecules at all possible adsorption sites of covalent organic framework-1 (COF-1) are studied by density functional theory (DFT) and second-order Moller−Plesset perturbation theory (MP2). The most favorable adsorption sites from our DFT results are on the top of an oxygen atom for the B3O3 ring and on the top of the center of the C−C bond for the benzene ring when a single H2 is interacting with the COF-1. The adsorption energy trend obtained from the DFT calculations is found to be in good agreement with the MP2 binding energy trend. The binding preferences are slightly changed when high hydrogen loading is considered. H2 molecules prefer to be trapped on the top of the carbon atoms of the benzene ring and also on the top of the oxygen atoms of the B3O3 ring. These findings are confirmed by hydrogen center-of-mass distribution results obtained using molecular dynamics simulations. Moreover, our DFT results reveal that the hydrogen adsorption energies are boosted when ...

Hydrogen Bonding Described Using Dispersion-Corrected Density Functional Theory

In recent works, dispersion-corrected atom-centered potentials (DCACPs) were developed as a method to account for long-range dispersion forces between molecules in density functional theory calculations within the generalized gradient approximation (GGA). Here, we test the ability of DCACPs to improve the GGA treatment of hydrogen-bonded systems. We assessed both BLYP and dispersion-corrected BLYP with respect to benchmark calculations for the hydrogen bond lengths and binding energies of 20 complexes containing the elements C, H, N, O, and S. Benchmark data included geometries calculated using MP2 and CCSD(T) and binding energies using W2, W1, CBS-QB3, and other CCSD(T) extrapolation schemes. With respect to benchmark methods, dispersion-corrected BLYP exhibited a mean signed error of 0.010 A in the hydrogen bond length and a mean relative error of 5.1% in the hydrogen bond binding energy. By comparison, uncorrected BLYP exhibited error statistics of 0.036 A and 15.9%, respectively. We conclude that DCACPs robustly improve the BLYP description of hydrogen-bonded systems at small additional computational cost. New benchmark geometries (MP2/aug-cc-pVTZ) and new benchmark binding energies (W1) are presented for seven complexes, and the remaining benchmark data were taken from previous literature.

Ab initio calculations on halogen‐bonded complexes and comparison with density functional methods

A systematic theoretical investigation on a series of dimeric complexes formed between some halocarbon molecules and electron donors has been carried out by employing both ab initio and density functional methods. Full geometry optimizations are performed at the Moller-Plesset second-order perturbation (MP2) level of theory with the Dunning's correlation-consistent basis set, aug-cc-pVDZ. Binding energies are extrapolated to the complete basis set (CBS) limit by means of two most commonly used extrapolation methods and the aug-cc-pVXZ (X = D, T, Q) basis sets series. The coupled cluster with single, double, and noniterative triple excitations [CCSD(T)] correction term, determined as a difference between CCSD(T) and MP2 binding energies, is estimated with the aug-cc-pVDZ basis set. In general, the inclusion of higher-order electron correlation effects leads to a repulsive correction with respect to those predicted at the MP2 level. The calculations described herein have shown that the CCSD(T) CBS limits yield binding energies with a range of -0.89 to -4.38 kcal/mol for the halogen-bonded complexes under study. The performance of several density functional theory (DFT) methods has been evaluated comparing the results with those obtained from MP2 and CCSD(T). It is shown that PBEKCIS, B97-1, and MPWLYP functionals provide accuracies close to the computationally very expensive ab initio methods.

An ab Initio Investigation of the Interactions Involving the Aromatic Group of the Set of Fluorinated N-(4-Sulfamylbenzoyl)benzylamine Inhibitors and Human Carbonic Anhydrase II

In this work we investigate the interactions that occur between the aromatic portion of the set of fluorinated N-(4-sulfamylbenzoyl)benzylamine (SBB) inhibitors and two residues of Human Carbonic Anhydrase II (HCAII), namely Phe-131 and Pro-202. Calculations were carried out at the MP2/aug-cc-pVDZ level of theory and the counterpoise scheme of Boys and Bernardi was employed to account for the basis set superposition error. The most striking result obtained here is that the SBB phenyl ring interacts at least as strongly with the proline pyrrolidine ring as with the phenylalanine phenyl ring, which is surprising because aromatic-aromatic interactions have long been thought to be particularly favorable in protein and protein-ligand structure. Comparison of the MP2 binding energies to those obtained with the Hartree-Fock method indicates that the attraction between the proline pyrrolidine ring and the SBB phenyl ring is largely attributable to dispersion forces. These favorable interactions between pyrrolidine and phenyl rings may have important implications in protein structure because there is potential for proline residues to interact with phenylalanine residues in a fashion analogous to that seen here. A preliminary protein data bank search indicates that the proline-phenylalanine contacts are about 40% as common as those between two phenylalanines. It is also found here that the number and pattern of fluorine substituents on the SBB phenyl ring is much less important in determining the SBB-HCAII binding energy than the relative geometric configuration of the interacting pairs.

Accurate ab Initio Study on the Hydrogen-Bond Pairs in Protein Secondary Structures.

Ab initio calculations up to the MP2/aug-cc-pVQZ//MP2/6-311+G** level have been carried out to characterize the four patterns of hydrogen-bond (H-bond) pairs in protein secondary structures. The unblocked and methyl-blocked glycine dipeptide dimers were arranged to model the H-bond pairs in α-helix (αHH) and antiparallel (Aββ-C5 and Aββ-C7) and parallel β-sheet (Pββ) secondary structures. The study uncovers that, in addition to the primary CO⋯NH H-bonds and the crossing secondary interactions, the CH⋯OC H-bonds and the tertiary effect (as we call it) also contribute substantially. The tertiary effect is due to the interpolarization between the donor and acceptor of a H-bond. This effect, which enhances the dipole-dipole interactions between two nearby H-bonds, stabilizes the β-sheet-like but destabilizes the helix-like H-bond pairs. The MP2 binding energies of the complexes were further refined by extrapolating to the complete basis set limit (CBS) according to Truhlar and co-workers and by a three-basis-set-based method. The best extrapolated CBS(aD-aT-aQ) binding energies of the unblocked dimers are -13.1 (αHH), -11.3 (Aββ-C5), -19.2 (Aββ-C7), and -14.8 kcal/mol (Pββ). For the methyl-blocked counterparts, the best extrapolated CBS(D-T-Q) binding energies are -14.8, -13.4, -20.8, and -16.7 kcal/mol, respectively. The interactions in the parallel β conformations are very close to the averages of the C5 and C7 antiparallel β conformations, and both are stronger than the helical dimers. Because the additive force fields are unable to account for the tertiary effect owing to the lack of polarization, all examined additive force fields significantly overestimate the interaction energies of the helix conformations relative to the β-sheet conformations. Notably, the agreement between molecular mechanical and quantum mechanical binding energies is improved after turning on the polarization. The study provides reference ab initio structures and binding energies for characterizing the backbone H-bonds of the protein secondary structures, which can be used for the parametrization of empirical molecular mechanics force fields.

Isomers of the Uracil Dimer: An ab Initio Benchmark Study

Benchmark ab initio calculations at the correlated level are reported for ten isomers of the uracil dimer (U.U): six are doubly N-H...O hydrogen bonded, three have a C-H...O and an N-H...O hydrogen bond, and one is doubly C-H...O hydrogen bonded. Their structures were optimized at the correlated level by using second-order Møller-Plesset perturbation theory (MP2), resolution of identity MP2 (RIMP2), and the binding energies D(e) calculated with the coupled-cluster method with singles, doubles, and iterative triples, CCSD(T). The MP2 and RIMP2 binding energies D(e) are extrapolated to the complete basis set (CBS) limit, using the aug-cc-pVXZ (X = D, T, Q) basis sets, giving binding energies accurate to +/-0.07 kcal/mol. With one exception, the correlation energy contributions at the CCSD(T) level increase the binding energies, although the changes are small, +0.03 to -0.27 kcal/mol (or 0.1% to 2.2%). The most stable isomer is the doubly N1-H...O hydrogen-bonded HB4 form, with D(e)[CCSD(T)]= -19.04 kcal/mol. The biologically relevant HB2 dimer has D(e)[CCSD(T)] = -12.64 kcal/mol, and the HB5 dimer that constitutes the main structural motif of the uracil crystal has -13.20 kcal/mol. The "Calcutta" dimer, which occurs in an RNA hexamer, is among the weakest isomers, with D(e)[CCSD(T)] = -9.81 kcal/mol. We compare to the binding energies calculated with the B3LYP, PBE, and PW91 density functionals; the PW91/6-311++G(d,p) binding energies agree with the CBS benchmark values, to within <2%. A useful single-molecule descriptor for the strengths of the individual hydrogen bonds can be derived from the gas-phase acidity DeltaE(0)(A-H) of the N-H or C-H donor groups and the gas-phase proton affinity PA(0)(B) of the C=O groups of the uracil monomer. The calculated hydrogen bond energies D(e)(infinity) correlate well with the difference between gas-phase acidity and basicity, DeltaE(0)(A-H) - PA(0)(B).

Binding energies of hydrogen molecules to isoreticular metal-organic framework materials

Recently, several novel isoreticular metal-organic framework (IRMOF) structures have been fabricated and tested for hydrogen storage applications. To improve our understanding of these materials, and to promote quantitative calculations and simulations, the binding energies of hydrogen molecules to the MOF have been studied. High-quality second-order Moller-Plesset (MP2) calculations using the resolution of the identity approximation and the quadruple zeta QZVPP basis set were used. These calculations use terminated molecular fragments from the MOF materials. For H2 on the zinc oxide corners, the MP2 binding energy using Zn4O(HCO2)6 molecule is 6.28 kJ/mol. For H2 on the linkers, the binding energy is calculated using lithium-terminated molecular fragments. The MP2 results with coupled-cluster singles and doubles and noniterative triples method corrections and charge-transfer corrections are 4.16 kJ/mol for IRMOF-1, 4.72 kJ/mol for IRMOF-3, 4.86 kJ/mol for IRMOF-6, 4.54 kJ/mol for IRMOF-8, 5.50 and 4.90 kJ/mol for IRMOF-12, 4.87 and 4.84 kJ/mol for IRMOF-14, 5.42 kJ/mol for IRMOF-18, and 4.97 and 4.66 kJ/mol for IRMOF-993. The larger linkers are all able to bind multiple hydrogen molecules per side. The linkers of IRMOF-12, IRMOF-993, and IRMOF-14 can bind two to three, three, and four hydrogen molecules per side, respectively. In general, the larger linkers have the largest binding energies, and, together with the enhanced surface area available for binding, will provide increased hydrogen storage. We also find that adding up NH2 or CH3 groups to each linker can provide up to a 33% increase in the binding energy.

Ab Initio Benchmark Study of (2-Pyridone)2, a Strongly Bound Doubly Hydrogen-Bonded Dimer

The 2-pyridone dimer, (2PY)2, has two antiparallel N−H···O H-bonds analogous to nucleobase dimers. The gas-phase rotational constants and all six intermolecular vibrational frequencies of (2PY)2 have been previously measured, providing benchmarks for theory. The structure, rotational constants, vibrational frequencies, and binding and dissociation energies of (2PY)2 were calculated at the correlated level using second-order Møller−Plesset perturbation theory (MP2) with medium to very large basis sets. The MP2 binding energy limit was extrapolated to the complete basis set (CBS) as De,CBS = −22.62 ± 0.07 kcal/mol. Higher order correlation energy contributions to De at the CCSD(T) level are destabilizing (+0.77 kcal/mol). This implies that (2PY)2 is the most strongly bound doubly hydrogen-bonded dimer known so far. The Hartree−Fock contribution to De,CBS is only ≈65%. Several medium-size basis sets yield MP2 De's within ±5% of the CBS value, as well as structure, rotational constants, and intermolecular vibrations in good agreement with experiment. The PW91 density functional method also shows very good performance with regard to all properties calculated, comparable to MP2. The results imply that correlated methods combined with carefully chosen medium-size basis sets may give near-quantitative results for the structures, binding energies, and intermolecular vibrational frequencies of nucleic acid base dimers.

Estimates of the ab initio limit for pi-pi interactions: the benzene dimer.

State-of-the-art electronic structure methods have been applied to the simplest prototype of aromatic pi-pi interactions, the benzene dimer. By comparison to results with a large aug-cc-pVTZ basis set, we demonstrate that more modest basis sets such as aug-cc-pVDZ are sufficient for geometry optimizations of intermolecular parameters at the second-order Møller-Plesset perturbation theory (MP2) level. However, basis sets even larger than aug-cc-pVTZ are important for accurate binding energies. The complete basis set MP2 binding energies, estimated by explicitly correlated MP2-R12/A techniques, are significantly larger in magnitude than previous estimates. When corrected for higher-order correlation effects via coupled cluster with singles, doubles, and perturbative triples [CCSD(T)], the binding energies D(e) (D(0)) for the sandwich, T-shaped, and parallel-displaced configurations are found to be 1.8 (2.0), 2.7 (2.4), and 2.8 (2.7) kcal mol(-1), respectively.

Accurate Structures and Binding Energies for Stacked Uracil Dimers

The face-to-face and face-to-back stacked uracil dimers have been investigated by second-order Moller−Plesset (MP2) perturbation theory and by the coupled-cluster singles and doubles method augmented with a perturbative contribution from connected triple substitutions [CCSD(T)]. Full MP2 geometry optimizations were performed with a TZ2P(f,d)++ basis and with the 6-31G* basis for which harmonic vibrational frequencies were computed as well. Complete basis set MP2 binding energies were obtained from basis set extrapolations using the correlation-consistent basis sets cc-pVXZ (X = D−5) and aug-cc-pVXZ (X = D−Q). Higher-order correlation effects were gauged by computing the MP2 → CCSD(T) shift in the counterpoise-corrected binding energy using a modified 6-31G* basis set. By adding this correction to the infinite basis set limit MP2 binding energies, final estimates of 9.7 and 8.8 kcal mol-1 are obtained for the binding energies of the face-to-face and face-to-back structures, respectively.