Computational studies of nonstoichiometric sodium auride clusters

The molecular structures of low-lying isomers of anionic and neutral sodium auride clusters have been studied computationally at the second-order M?ller-Plesset perturbation theory level using quadruple-ζ basis sets augmented with a double set of polarization functions. The first vertical detachment energies were calculated at the M?ller-Plesset level as the energy difference between the cluster anion and the corresponding neutral cluster. The photodetachment energies of higher-lying ionization channels were calculated by adding electronic excitation energies of the neutral clusters to the first vertical detachment energy. The excitation energies were calculated at the linear response approximate coupled-cluster singles and doubles level using the anionic cluster structures. The obtained ionization energies for NaAu(-), NaAu(2)(-), NaAu(3)(-), NaAu(4)(-), Na(2)Au(2)(-), Na(2)Au(3)(-), Na(3)Au(3)(-), and Na(2)Au(4)(-) were compared to values deduced from experimental photoelectron spectra. Comparison of the calculated photoelectron spectra for a few energetically low-lying isomers shows that the energetically lowest cluster structures obtained in the calculations do not always correspond to the clusters produced experimentally. Spin-component-scaled second-order M?ller-Plesset perturbation theory calculations shift the order of the isomers such that the observed clusters more often correspond to the energetically lowest structure, whereas the spin-component-scaled approach does not improve the photodetachment energies of the sodium aurides. The potential energy surface of the sodium aurides is very soft, with several low-lying isomers requiring an accurate electron correlation treatment. The calculations show that merely the energetic criterion is not a reliable means to identify the structures of the observed sodium auride clusters; other experimental information is needed to ensure a correct assignment of the cluster structures. The cluster structures of nonstoichiometric anionic sodium aurides have been determined by comparing calculated ionization energies for low-lying structures of the anionic clusters with experimental data.     

Extending the molecular size in accurate quantum-chemical calculations: the equilibrium structure and spectroscopic properties of uracil

The equilibrium structure of uracil has been investigated using both theoretical and experimental data. With respect to the former, quantum-chemical calculations at the coupled-cluster level in conjunction with a triple-zeta basis set have been carried out. Extrapolation to the basis set limit, performed employing the second-order M?ller-Plesset perturbation theory, and inclusion of core-correlation and diffuse-function corrections have also been considered. Based on the available rotational constants for various isotopic species together with corresponding computed vibrational corrections, the semi-experimental equilibrium structure of uracil has been determined for the first time. Theoretical and semi-experimental structures have been found in remarkably good agreement, thus pointing out the limitations of previous experimental determinations. Molecular and spectroscopic properties of uracil have then been studied by means of the composite computational approach introduced for the molecular structure evaluation. Among the results achieved, we mention the revision of the dipole moment. On the whole, it has been proved that the computational procedure presented is able to provide parameters with the proper accuracy to support experimental investigations of large molecules of biological interest.     

High-level ab initio studies of hydrogen abstraction from prototype hydrocarbon systems

Symmetric and nonsymmetric hydrogen abstraction reactions are studied using state-of-the-art ab initio electronic structure methods. Second-order M?ller-Plesset perturbation theory (MP2) and the coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] methods with large correlation consistent basis sets (cc-pVXZ, where X = D,T,Q) are used in determining the transition-state geometries, activation barriers, and thermodynamic properties of several representative hydrogen abstraction reactions. The importance of basis set, electron correlation, and choice of zeroth-order reference wave function in the accurate prediction of activation barriers and reaction enthalpies are also investigated. The ethynyl radical (*CCH), which has a very high affinity for hydrogen atoms, is studied as a prototype hydrogen abstraction agent. Our high-level quantum mechanical computations indicate that hydrogen abstraction using the ethynyl radical has an activation energy of less than 3 kcal mol(-1) for hydrogens bonded to an sp(2) or sp(3) carbon. These low activation barriers further corroborate previous studies suggesting that ethynyl-type radicals would make good tooltips for abstracting hydrogens from diamondoid surfaces during mechanosynthesis. Modeling the diamond C(111) surface with isobutane and treating the ethynyl radical as a tooltip, hydrogen abstraction in this reaction is predicted to be barrierless.     

On the nature of the Møller-Plesset critical point

It has been suggested [F. H. Stillinger, J. Chem. Phys. 112, 9711 (2000)] that the convergence or divergence of M?ller-Plesset perturbation theory is determined by a critical point at a negative value of the perturbation parameter z at which an electron cluster dissociates from the nuclei. This conjecture is examined using configuration-interaction computations as a function of z and using a quadratic approximant analysis of the high-order perturbation series. Results are presented for the He, Ne, and Ar atoms and the hydrogen fluoride molecule. The original theoretical analysis used the true Hamiltonian without the approximation of a finite basis set. In practice, the singularity structure depends strongly on the choice of basis set. Standard basis sets cannot model dissociation to an electron cluster, but if the basis includes diffuse functions then it can model another critical point corresponding to complete dissociation of all the valence electrons. This point is farther from the origin of the z plane than is the critical point for the electron cluster, but it is still close enough to cause divergence of the perturbation series. For the hydrogen fluoride molecule a critical point is present even without diffuse functions. The basis functions centered on the H atom are far enough from the F atom to model the escape of electrons away from the fluorine end of the molecule. For the Ar atom a critical point for a one-electron ionization, which was not previously predicted, seems to be present at a positive value of the perturbation parameter. Implications of the existence of critical points for quantum-chemical applications are discussed.     

Accurate ab initio structure, dissociation energy, and vibrational spectroscopy of the F(-)-CH4 anion complex

Accurate equilibrium structure, dissociation energy, global potential energy surface (PES), dipole moment surface (DMS), and the infrared vibrational spectrum in the 0-3000 cm(-1) range of the F(-)-CH4 anion complex have been obtained. The equilibrium electronic structure calculations employed second-order M?ller-Plesset perturbation theory (MP2) and coupled-cluster (CC) method up to single, double, triple, and perturbative quadruple excitations using the aug-cc-p(C)VXZ [X = 2(D), 3(T), 4(Q), and 5] correlation-consistent basis sets. The best equilibrium geometry has been obtained at the all-electron CCSD(T)/aug-cc-pCVQZ level of theory. The dissociation energy has been determined based on basis set extrapolation techniques within the focal-point analysis (FPA) approach considering (a) electron correlation beyond the all-electron CCSD(T) level, (b) relativistic effects, (c) diagonal Born-Oppenheimer corrections (DBOC), and (d) variationally computed zero-point vibrational energies. The final D(e) and D0 values are 2398 +/- 12 and 2280 +/- 20 cm(-1), respectively. The global PES and DMS have been computed at the frozen-core CCSD(T)/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels of theory, respectively. Variational vibrational calculations have been performed for CH4 and F(-)-CH4 employing the vibrational configuration interaction (VCI) method as implemented in Multimode.     

Quantum-chemical investigation of the structures and electronic spectra of the nucleic acid bases at the coupled cluster CC2 level

We study the ground-state structures and singlet- and triplet-excited states of the nucleic acid bases by applying the coupled cluster model CC2 in combination with a resolution-of-the-identity approximation for electron interaction integrals. Both basis set effects and the influence of dynamic electron correlation on the molecular structures are elucidated; the latter by comparing CC2 with Hartree-Fock and M?ller-Plesset perturbation theory to second order. Furthermore, we investigate basis set and electron correlation effects on the vertical excitation energies and compare our highest-level results with experiment and other theoretical approaches. It is shown that small basis sets are insufficient for obtaining accurate results for excited states of these molecules and that the CC2 approach to dynamic electron correlation is a reliable and efficient tool for electronic structure calculations on medium-sized molecules.     

Computational study of hydrogen binding by metal-organic framework-5

We report the results of quantum chemistry calculations on H(2) binding by the metal-organic framework-5 (MOF)-5. Density functional theory calculations were used to calculate the atomic positions, lattice constant, and effective atomic charges from the electrostatic potential for the MOF-5 crystal structure. Second-order M?ller-Plesset perturbation theory was used to calculate the binding energy of H(2) to benzene and H(2)-1,4-benzenedicarboxylate-H(2). To achieve the necessary accuracy, the large Dunning basis sets aug-cc-pVTZ, and aug-cc-pVQZ were used, and the results were extrapolated to the basis set limit. The binding energy results were 4.77 kJ/mol for benzene, 5.27 kJ/mol for H(2)-1,4-benzenedicarboxylate-H(2). We also estimate binding of 5.38 kJ/mol for Li-1,4-benzenedicarboxylate-Li and 6.86 kJ/mol at the zinc oxide corners using second-order M?ller-Plesset perturbation theory. In order to compare our theoretical calculations to the experimental hydrogen storage results, grand canonical Monte Carlo calculations were performed. The Monte Carlo simulations identify a high energy binding site at the corners that quickly saturated with 1.27 H(2) molecules at 78 K. At 300 K, a broad range of binding sites are observed.     

Photochemistry of water: the (H2O)5 cluster

The structures of the cyclic water pentamer, the H3O+(H2O)3OH- zwitterion, and the H3O(H2O)3OH biradical form of the (H2O)5 cluster have been determined with the second-order M?ller-Plesset method and with density-functional theory (DFT). The vertical singlet excitation energies of these structures have been calculated with the second-order approximated coupled-cluster method and with time-dependent DFT, respectively. The molecular and electronic structures of the H3O(H2O)3OH biradical have been characterized for the first time. The lowest electronic states of the biradical are slightly lower in energy than the vertically excited states of the covalent and zwitterionic (H2O)5 clusters and therefore are photochemically accessible from the latter. The electronic absorption spectrum of the biradical exhibits the characteristic features of the absorption spectrum of the hydrated electron. It is argued that the basic mechanisms of the photochemistry of water, in particular the generation of the hydrated electron by UV photons, can be unraveled by relatively straightforward electronic structure and dynamics calculations for finite-size water clusters.     

Can a dipole-bound electron form a pseudo-atom? An atoms-in-molecules study of the hydrated electron

Non-nuclear local maxima, or attractors, of electron density are a rare but very interesting feature of the electron density distribution in molecules and solids. Recently, non-nuclear attractors (NNAs) and the corresponding pseudoatoms of electron density have been identified with the quantum theory of atoms in molecules for some anionic clusters formed by several polar solvent molecules and an excess electron bound in either a solvated-electron or dipole-bound fashion. This contribution reports a detailed study of the topology of the electron density for a series of dipole-bound water cluster anions, as calculated with Hartree-Fock, M?ller-Plesset perturbation theory, and coupled-cluster methods together with basis sets augmented with extra diffuse basis functions to accommodate the excess electron. For dipole-bound clusters, electron densities obtained with insufficient inclusion of electron correlation effects and tight basis sets feature a well-pronounced pseudoatom due to the excess electron, which ultimately disappears when a higher level of electronic structure theory and a more diffuse basis set are used. On the other hand, for solvated-electron clusters, where the excess electron is surrounded by solvent molecules, the existence of NNAs does not seem to be an artifact of the method employed, but rather a genuine feature of the electron density distribution. Pseudoatoms of electron density thus appear to be an exclusive feature of confined environments and are unlikely to be found on the tip of a cluster dipole or on solid surfaces.     

Competition between van der Waals and hydrogen bonding interactions: structure of the trans-1-naphthol/N(2) cluster

The excitation energy in the multiphoton ionization spectrum of the trans-1-naphthol/N(2) cluster shows only a small red shift with respect to isolated naphthol, indicating a van der Waals pi-bound structure rather than a hydrogen-bonded one. To confirm this interpretation, high-level electronic structure calculations were performed for several pi- and hydrogen-bonded isomers of this cluster. The calculations were carried out at the second order M?ller-Plesset (MP2) level of perturbation theory with the family of correlation consistent basis sets up to quintuple-zeta quality including corrections for the basis set superposition error and extrapolation to the MP2 complete basis set (CBS) limit. We report the optimal geometries, vibrational frequencies, and binding energies (D(e)), also corrected for harmonic zero-point energies (D(0)), for three energetically low-lying isomers. In all calculations the lowest energy structure was found to be an isomer with the N(2) molecule bound to the pi-system of the naphthol ring carrying the OH group. In the CBS limit its dissociation energy was computed to be D(0) = 2.67 kcal/mol (934 cm(-1)) as compared to D(0) = 1.28 kcal/mol (448 cm(-1)) for the H-bound structure. The electronic structure calculations therefore confirm the assignment of the experimental electronic spectrum corresponding to a van der Waals pi-bound structure. The energetic stabilization of the pi-bound isomer with respect to the hydrogen-bonded one is rather unexpected when compared with previous findings in related systems, in particular phenol/N(2).     

Introducing phase transitions to quantum chemistry: from Trouton's rule to first principles vaporization entropies

In the present study, we employ quantum cluster equilibrium calculations on a small water cluster set in order to derive thermochemical equilibrium properties of the liquid phase as well as the liquid-vapor phase transition. The focus is set on the calculation of liquid phase entropies, from which entropies of vaporization at the normal boiling point of water are derived. Different electronic structure methods are compared and the influences of basis set size and of cooperative effects are discussed. In line with a previous study on the subject [B. Kirchner, J. Chem. Phys. 123, 204116 (2005)], we find that the neglect of cooperativity leads to large errors in the equilibrium cluster populations as well as in the obtained entropy values. In contrast, a correct treatment of the intermolecular many-body interaction yields liquid phase entropies and phase transition entropies being in very good agreement with the experimental reference, thus demonstrating that the quantum cluster equilibrium partition function intrinsically accounts for the shortcomings of the ideal gas partition function often employed in first principles entropy calculations. Comparing the calculated vaporization entropies to the value predicted by Trouton's rule, it is observed that for entropy calculations the consideration of intracluster cooperative effects is more important than the explicit treatment of the intercluster association even in a highly associated liquid such as water. The decomposition of entropy into contributions due to different degrees of freedom implies the need for the accurate treatment of particle indistinguishability and free volume of translation, whereas minor influences should be expected from the vibrational and rotational degrees of freedom and none from the electronic degrees of freedom.     

Nonempirical studies of the molecular properties and thermodynamic functions of urea in the ideal gas state

Quantum-chemical calculations were performed for the equilibrium structure of isolated urea molecules using the 6-311++G** basis set and second-order M?ller-Plesset perturbation theory, density functional theory, and the coupled cluster method with the local inclusion of electron correlation. The results were used to perform statistical calculations of the standard thermodynamic functions of urea in the ideal gas state taking into account anharmonicity of normal vibrations. The contributions of dimerization and isomerization of urea molecules in the vapor phase were determined. The recommended values were compared with the available experimental data and the results of preceding calculations. Original Russian Text ? A.V. Kuznetsov, A.V. Stolyarov, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 2, pp. 339–345.     

Calculation of electron detachment energies for water cluster anions: an appraisal of electronic structure methods, with application to (H2O)20- AND (H2O)24-

We present benchmark calculations of vertical electron detachment energies (VDEs) for various conformers of (H2O)n-, using both wave function and density functional methods, in sequences of increasingly diffuse Gaussian basis sets. For small clusters (n < or = 6), a systematic examination of VDE convergence reveals that it is possible to converge this quantity to within approximately 0.01 eV of the complete-basis limit, using a highly diffuse but otherwise economical Pople-style basis set of double-zeta quality, with 28 atom-centered basis functions per water molecule. Floating-center basis functions can be useful but are not required to obtain accurate VDEs. Second-order M?ller-Plesset perturbation (MP2) theory suffices to obtain VDEs that are within 0.05 eV of the results from both experiment and coupled-cluster theory, and which always err toward underbinding the extra electron. In contrast to these consistent predictions, VDEs calculated using density functional theory (DFT) vary widely, according to the fraction of Hartree-Fock exchange in a given functional. Common functionals such as BLYP and B3LYP overestimate the VDE by 0.2-0.5 eV, whereas a variant of Becke's "half and half" functional is much closer to coupled-cluster predictions. Exploratory calculations for (H2O)20- and (H2O)24- cast considerable doubt on earlier calculations that were used to assign the photoelectron spectra of these species to particular cluster isomers.     

Interaction energy contributions of H-bonded and stacked structures of the AT and GC DNA base pairs from the combined density functional theory and intermolecular perturbation theory approach

Stacked and Watson-Crick structures of DNA base pairs are investigated with the DFT-SAPT variant of intermolecular perturbation theory, yielding a rigorous decomposition of the interaction energy into electrostatic, induction, dispersion, and exchange contributions. Their interplay in the various structures is analyzed. Total interaction energies extrapolated to the complete basis set limit are compared with corresponding second-order M?ller-Plesset and estimated coupled-cluster theory results.     

Accelerating resolution-of-the-identity second-order Møller-Plesset quantum chemistry calculations with graphical processing units

The modification of a general purpose code for quantum mechanical calculations of molecular properties (Q-Chem) to use a graphical processing unit (GPU) is reported. A 4.3x speedup of the resolution-of-the-identity second-order M?ller-Plesset perturbation theory (RI-MP2) execution time is observed in single point energy calculations of linear alkanes. The code modification is accomplished using the compute unified basic linear algebra subprograms (CUBLAS) library for an NVIDIA Quadro FX 5600 graphics card. Furthermore, speedups of other matrix algebra based electronic structure calculations are anticipated as a result of using a similar approach.     

Simple empirical formulas and good quality estimations for electron correlation energies of molecules and molecular clusters: First-row atom molecules

Using the ab initio coupled cluster method, correlation energies were calculated for a number of molecules composed of first-row atoms. The results of computations can be fitted rather well with simple analytic formulas. The main result of the present investigation is that intraatomic part of the correlation energy is proportional to sum of squares of valence electron charges on atoms composing the molecule. The proportionality coefficient depends on the basis set used. Independently, the approximations were introduced which allow for good estimation of intraatomic correlation energy by using M øller-Plesset second-order perturbation calculations only. © 1996 John Wiley & Sons, Inc.     

An ab initio study of van der Waals potential energy parameters for silver clusters

We employ ab initio calculations of van der Waals complexes to study the potential energy parameters (C(6) coefficients) of van der Waals interactions for modeling of the adsorption of silver clusters on the graphite surface. Electronic structure calculations of the (Ag(2))(2), Ag(2)-H(2), and Ag(2)-C(6)H(6) complexes are performed using a coupled-cluster approach that includes single, double, and perturbative triple excitations (CCSD(T)), M?ller-Plesset second-order perturbation theory (MP2), and spin-component-scaled MP2 (SCS-MP2) methods. Using the atom pair approximation, the C(6) coefficients for silver-silver, silver-hydrogen, and silver-carbon atom systems are obtained after subtracting the energies of quadrupole-quadrupole interactions from the total electronic energy.     

Coupled-cluster methods with perturbative inclusion of explicitly correlated terms: a preliminary investigation

We propose to account for the large basis-set error of a conventional coupled-cluster energy and wave function by a simple perturbative correction. The perturbation expansion is constructed by L?wdin partitioning of the similarity-transformed Hamiltonian in a space that includes explicitly correlated basis functions. To test this idea, we investigate the second-order explicitly correlated correction to the coupled-cluster singles and doubles (CCSD) energy, denoted here as the CCSD(2)(R12) method. The proposed perturbation expansion presents a systematic and easy-to-interpret picture of the "interference" between the basis-set and correlation hierarchies in the many-body electronic-structure theory. The leading-order term in the energy correction is the amplitude-independent R12 correction from the standard second-order M?ller-Plesset R12 method. The cluster amplitudes appear in the higher-order terms and their effect is to decrease the basis-set correction, in accordance with the usual experience. In addition to the use of the standard R12 technology which simplifies all matrix elements to at most two-electron integrals, we propose several optional approximations to select only the most important terms in the energy correction. For a limited test set, the valence CCSD energies computed with the approximate method, termed , are on average precise to (1.9, 1.4, 0.5 and 0.1%) when computed with Dunning's aug-cc-pVXZ basis sets [X = (D, T, Q, 5)] accompanied by a single Slater-type correlation factor. This precision is a roughly an order of magnitude improvement over the standard CCSD method, whose respective average basis-set errors are (28.2, 10.6, 4.4 and 2.1%). Performance of the method is almost identical to that of the more complex iterative counterpart, CCSD(R12). The proposed approach to explicitly correlated coupled-cluster methods is technically appealing since no modification of the coupled-cluster equations is necessary and the standard M?ller-Plesset R12 machinery can be reused.     

Are insertion compounds of CH2CHF and the rare gases stable? A computational study

Ab initio calculations, using second-order M?ller-Plesset perturbation theory with a 6-311+ +G(2d,2p) basis set, predict the stability of two novel compounds of monofluoroethene, CH2CHF, with the rare-gas atoms Ar and Kr. The dissociation energies to the lowest-energy fragmentation products, CH2CHF + Rg (Rg = Ar,Kr), were computed to be -528 and -449 kJ mol(-1), respectively, at the coupled-cluster singles, doubles, and triples level of theory. Possible transition states (at second-order M?ller-Plesset theory) via a C-Rg-F bending mode for these fragmentation reactions were also located with barrier heights of about 76 and 106 kJ mol(-1), for the Ar- and Kr-containing species, respectively. However, the Ar-containing species may not exist at all as it is less stable than the fragments CH2CH + F + Ar at the higher level of theory and may possibly dissociate via this route.