•Brueckner orbitals are used as a trial wave function for diffusion Monte Carlo calculations.•For neutral systems, Brueckner orbitals result in a lower DMC energy than Hartree–Fock orbitals.•For a CO2 anion, DMC energies are lowest when a Brueckner trial wave function is used.This study explores the utility of Brueckner orbitals as trial wave functions for diffusion Monte Carlo (DMC) calculations. Comparison is made with Hartree–Fock (HF) and density functional theory (DFT) orbitals allowing for an assessment of how well the three sets of orbitals describe the nodal surfaces. For the neutral test systems, PBE0 orbitals or Brueckner orbitals give DMC energies that are appreciably lower than those obtained using Hartree–Fock orbitals. For a CO2− anion test case, a significantly lower DMC energy is obtained when using Brueckner orbitals rather than DFT orbitals as the trial function.

MP2 and symmetry-adapted perturbation theory calculations are used in conjunction with the aug-cc-pVQZ basis set to characterize the SF6 dimer. Both theoretical methods predict the global minimum structure to be of C2 symmetry, lying 0.07–0.16 kJ/mol below a C2h saddle point structure, which, in turn, is predicted to lie energetically 0.4–0.5 kJ/mol below the lowest-energy D2d structure. This is in contrast with IR spectroscopic studies that infer an equilibrium D2d structure. It is proposed that the inclusion of vibrational zero-point motion gives an averaged structure of D2d symmetry.

Symmetry-adapted perturbation theory (SAPT) calculations are carried out to elucidate the intermolecular interactions present between fluorinated and nonfluorinated alkyl chain groups and aromatic π systems in the folded and unfolded conformers of Wilcox torsion balance systems. The calculations predict the folded conformers to be 2.0–2.3 kcal/mol more stable than the unfolded conformers, with the preference for the folded conformer being greater in the fluorinated alkyl chain case. We also establish that a simple electrostatic analysis, based on atomic charges, is inadequate for understanding the conformational preferences of these systems. In the folded conformers, there are sizable charge penetration contributions that are not recovered by point charge models. Additionally, the SAPT analysis reveals that exchange–repulsion interactions make a significant contribution to the relative stability of the folded and unfolded conformer.

We review the role that gas-phase, size-selected protonated water clusters, H+(H2O)n, have played in unraveling the microscopic mechanics responsible for the spectroscopic behavior of the excess proton in bulk water. Because the larger (n ≥ 10) assemblies are formed with three-dimensional cage morphologies that more closely mimic the bulk environment, we report the spectra of cryogenically cooled (10 K) clusters over the size range 2 ≤ n ≤ 28, over which the structures evolve from two-dimensional arrangements to cages at around n = 10. The clusters that feature a complete second solvation shell around a surface-embedded hydronium ion yield spectral signatures of the proton defect similar to those observed in dilute acids. The origins of the large observed shifts in the proton vibrational signature upon cluster growth were explored with two types of theoretical analyses. First, we calculate the cubic and semidiagonal quartic force constants and use these in vibrational perturbation theory calculations to establish the couplings responsible for the large anharmonic red shifts. We then investigate how the extended electronic wave functions that are responsible for the shapes of the potential surfaces depend on the nature of the H-bonded networks surrounding the charge defect. These considerations indicate that, in addition to the sizable anharmonic couplings, the position of the OH stretch most associated with the excess proton can be traced to large increases in the electric fields exerted on the embedded hydronium ion upon formation of the first and second solvation shells. The correlation between the underlying local structure and the observed spectral features is quantified using a model based on Badger’s rule as well as via the examination of the electric fields obtained from electronic structure calculations.

In this work, we characterize the nonvalence correlation-bound anion states of several polycyclic aromatic hydrocarbon (PAH) molecules. Unlike the analogous image potential states of graphene that localize the charge density of the excess electron above and below the plane of the sheet, we find that for PAHs, much of the charge distribution of the excess electron is localized around the periphery of the molecule. This is a consequence of the electrostatic interaction of the electron with the polar CH groups. By replacing the H atoms by F atoms or the CH groups by N atoms, the charge density of the excess electron shifts from the periphery to above and below the plane of the ring systems.

We present a one-electron model Hamiltonian for characterizing nonvalence correlation-bound anion states of fullerene molecules. These states are the finite system analogs of image potential states of metallic surfaces. The model potential accounts for both atomic and charge-flow polarization and is used to characterize the nonvalence correlation-bound anion states of the C60, (C60)2, C240, and C60@C240 fullerene systems. Although C60 is found to have a single (s-type) nonvalence correlation-bound anion state, the larger fullerenes are demonstrated to have multiple nonvalence correlation-bound anion states.

The ground-state anion of perfluorobenzene is investigated by means of equation-of-motion (EOM) methods. It is found that at the geometry of the neutral, the excess electron is bound by 0.135 eV. This anion state is nonvalence in nature with the excess electron bound in a very diffuse orbital with dispersion-type interactions between the excess electron and the valence electrons being pivotal to the binding. The diffuse correlation-bound state is shown to evolve into a more stable compact valence-bound anion state with a buckled structure having an adiabatic EA of 0.5 eV. Results are also presented for the bound anion states of the C6F6 dimer.

The vibrational spectroscopy of the nitrate-water isotopologues is studied in the O–H and O–D stretching regions using temperature-dependent infrared multiple photon dissociation spectroscopy combined with calculations of the anharmonic spectra. At a temperature of 15 K a series of discrete peaks is observed in the IRMPD spectra of NO3–·H2O, NO3–·HDO, and NO3–·D2O. This structure is considerably more complex than predicted by harmonic calculations. A signal is only observed in the hydrogen-bonded O–H (O–D) stretching region, characteristic of a double hydrogen-bond donor binding motif. With increasing temperature, the peaks broaden, leading to a quasi-continuous absorption from 3150 to 3600 cm–1 (2300–2700 cm–1) for NO3–·H2O (NO3–·D2O) and, above 100 K, an additional band in the free O–H (O–D) stretching region, suggesting the population of complexes containing only a single hydrogen bond at higher internal energies. Vibrational configuration interaction calculations confirm that much of the structure observed in the IRMPD spectra derives from progressions in the water rocking mode resulting from strong cubic coupling between the O–H (O–D) stretch and water rock degrees of freedom. The spectra of both NO3–·H2O and NO3–·D2O display a strong peak that does not derive from the water rock progression but results instead from a Fermi resonance between the O–H (O–D) stretch and H–O–H (D−O–D) bend overtone. Additional insight into the nature of the O–H (O–D) stretch and water rocking coupling in these complexes is provided by an effective Hamiltonian that allows for the cubic coupling between the O–H stretch and water rock degrees of freedom.

The nature of the electronic ground state of the tetramethyleneethane (TME) diradical has proven to be a challenge for both experiment and theory. Through the use of quantum Monte Carlo (QMC) methods and multireference perturbation theory, we demonstrate that the lowest singlet state of TME is energetically lower than the lowest triplet state at all values of the torsional angle between the allyl subunits. Moreover, we find that the maximum in the potential energy curve for the singlet state occurs at a torsional angle near 45°, in contrast to previous calculations that placed the planar structure of the singlet state as the highest in energy. We also show that the CASPT2 method when used with a sufficiently large reference space and a sufficiently flexible basis set gives potential energy curves very close to those from the QMC calculations. Our calculations have converged the singlet–triplet gap of TME as a function of methodology and basis set. These results provide insight into the level of theory required to properly model diradicals, in particular disjoint diradicals, and provide guidelines for future studies on more complicated diradical systems.

In this paper, we introduce correlation consistent Gaussian-type orbital basis sets for the H and B–Ne atoms for use with the CASINO Dirac–Fock AREP pseudopotentials. These basis sets are tested in coupled cluster calculations on H2, B2, C2, N2, O2, and F2 as well as in quantum Monte Carlo calculations on the water monomer and dimer and the water–benzene complex, where they are found to give low variances in variational Monte Carlo calculations and to lead to reduced time step errors and improved convergence in diffusion Monte Carlo calculations compared to the use of nonoptimized basis sets. The use of basis sets with a large number of contracted s and p primitives is found to be especially important for the convergence of the energy in the diffusion Monte Carlo calculations.

It is established using high-level electronic structure calculations that C60 has an s-type correlation-bound anion state with an electron binding energy of about 120 meV. Examination of the “singly occupied” natural orbital of the anion reveals that about 9% of the charge density of the excess electron is localized inside, and about 91% is localized outside the C60 cage. Calculations were also carried out for the He@C60, Ne@C60, and H2O@C60 endohedral complexes. For each of these species, the s-type anion is predicted to be less strongly bound than for C60 itself.Keywords: correlation-bound anion; endohedral; equation-of-motion; fullerenes; Rydberg electron transfer;

A new polarization model potential for describing the interaction of an excess electron with water clusters is presented. This model, which allows for self-consistent electron–water and water–water polarization, including dispersion interactions between the excess electron and the water monomers, gives electron binding energies in excellent agreement with high-level ab initio calculations for both surface-bound and cavity-bound states of (H2O)n– clusters. By contrast, model potentials that do not allow for a self-consistent treatment of electron–water and water–water polarization are less successful at predicting the relative stability of surface-bound and cavity-bound excess electron states.

The Tkatchenko–Scheffler vdW-TS method [Phys. Rev. Lett.2009, 102, 073005] has been implemented in a plane-wave DFT code and used to characterize several dispersion-dominated systems, including layered materials, noble-gas solids, and molecular crystals. Full optimizations of the structures, including relaxation of the stresses on the unit cells, were carried out. Internal geometrical parameters, lattice constants, bulk moduli, and cohesive energies are reported and compared to experimental results.

State-of-the-art ADC(2), EOM-EA-CCSD, and EOM-EA-CCSD(2) many-body methods are used to calculate the energies for binding an excess electron to selected water clusters up to (H2O)24 in size. The systems chosen for study include several clusters for which the Hartree–Fock method either fails to bind the excess electron or binds it only very weakly. The three theoretical methods are found to give similar values of the electron binding energies. The reported electron binding energies are the most accurate to date for such systems, and these results should prove especially valuable as benchmarks for testing model potential approaches for describing the interactions of excess electrons with water clusters and bulk water.

The interaction of a water molecule with the (100) surface of MgO as described by cluster models is studied using MP2, coupled MP2 (MP2C) and symmetry–adapted perturbation theory (SAPT) methods. In addition, diffusion Monte Carlo (DMC) results are presented for several slab models as well as for the smallest, 2X2 cluster model. For the 2X2 model it is found that the MP2C, DMC, and CCSD(T) methods give nearly the same potential energy curve for the water–cluster interaction, whereas the potential energy curve from the SAPT calculations differs slightly from those of the other methods. The interaction of the water molecule with the cluster models of the MgO(100) surface is weakened upon expanding the number of layers from one to two and also upon expanding the description of the layers from 2X2 to 4X4 to 6X6. The SAPT calculations reveal that both these expansions of the cluster model are accompanied by reductions in the magnitudes of the induction and dispersion constributions. The best estimate of the energy for binding an isolated water molecule to the surface obtained from the cluster model calculations is in good agreement with that obtained from the DMC calculations using a 2–layer slab model with periodic boundary conditions.

The interaction of a water monomer with a series of linear acenes (benzene, anthracene, pentacene, heptacene, and nonacene) is investigated using a wide range of electronic structure methods, including several “dispersion”-corrected density functional theory (DFT) methods, several variants of the random phase approximation (RPA), DFT-based symmetry-adapted perturbation theory with density fitting (DF-DFT-SAPT), MP2, and coupled-cluster methods. The DF-DFT-SAPT calculations are used to monitor the evolution of the electrostatics, exchange-repulsion, induction, and dispersion contributions to the interaction energies with increasing acene size and also provide the benchmark data against which the other methods are assessed.

A discrete variable representation (DVR) implementation of an one-electron polarization model (OPEM) for characterizing (H2O)n− clusters is described. For the (H2O)90− cluster, evaluation of the energy and gradient using a suitable DVR basis sets is about a 2 orders of magnitude faster than corresponding calculations using a Gaussian orbital basis set. The DVR version of the code has been parallelized using OpenMP to enable molecular dynamics (MD) simulations of large (H2O)n− clusters.

In a previous study (J. Phys. Chem. C, 2009, 113, 10242–10248) we used density functional theory based symmetry–adapted perturbation theory (DFT–SAPT) calculations of water interacting with benzene (C6H6), coronene (C24H12), and circumcoronene (C54H18) to estimate the interaction energy between a water molecule and a graphene sheet. The present study extends this earlier work by use of a more realistic geometry with the water molecule oriented perpendicular to the acene with both hydrogen atoms pointing down. We also include results for an intermediate C48H18 acene. Extrapolation of the water–acene results gives a value of −3.0 ± 0.15 kcal mol−1 for the binding of a water molecule to graphene. Several popular dispersion-corrected DFT methods are applied to the water–acene systems and the resulting interacting energies are compared to results of the DFT–SAPT calculations in order to assess their performance.

Molecular dynamics simulations are used to characterize the hydrates of Xe, methane, and CO2, allowing for a systematic comparison of the structural and dynamical properties for these three hydrates. Although the host−guest interaction energy for the T = 0 K structures is most attractive in the case of Xe, other structural and dynamical properties from the simulations indicate that, in fact, host−guest coupling is most important for the CO2 hydrate. Specifically, the host lattice of CO2 hydrate expands more with increasing temperature than do the lattices of the xenon and methane hydrates, and the translational and rotational dynamics of the water molecules are predicted to be most perturbed in the CO2 hydrate. The simulations predict that the CO2 and xenon hydrates have lower speed of sound values and lower themal conductivities than methane hydrate or the empty lattice.

The low-lying potential energy minima of the H+(H2O)n, n = 6, 21, and 22, protonated water clusters have been investigated using two versions of the self-consistent-charge density-functional tight-binding plus dispersion (SCC-DFTB+D) electronic structure methods. The relative energies of different isomers calculated using the SCC-DFTB+D methods are compared with the results of DFT and MP2 calculations. This comparison reveals that for H+(H2O)6 the SCC-DFTB+D method with H-bonding and third-order corrections more closely reproduces the results of the MP2 calculations, whereas for the n = 21 and 22 clusters, the uncorrected SCC-DFTB+D method performs better. Both versions of the SCC-DFTB+D method are found to be biased toward Zundel structures.

The diffusion Monte Carlo (DMC) method is used to calculate the electron binding energies of two forms of (H2O)6−. It is found that the DMC method, when using either Hartree−Fock or density functional theory trial wave functions, gives electron binding energies in excellent agreement with the results of large basis set CCSD(T) calculations. This demonstrates that the DMC method will be a viable method for characterizing larger (H2O)n− ions for which CCSD(T) calculations are not feasible.

We report the spectral signatures of water molecules occupying individual sites in an extended H-bonding network using mass-selective, double-resonance vibrational spectroscopy of isotopomers. The scheme is demonstrated on the water heptamer anion, (H2O)7¯, where we first randomly incorporate a single, intact D2O molecule to create an ensemble of isotopomers. The correlation between the two OD stretching frequencies and that of the intramolecular DOD bending transition is then revealed by photochemical modulation of the isotopomer population responsible for particular features in the vibrational spectrum. The observed patterns confirm the assignment of the dominant doublet, appearing most red-shifted from the free OD stretch, to a single water molecule attached to the network in a double H-bond acceptor (AA) arrangement. The data also reveal the unanticipated role of accidentally overlapping transitions, where the highest-energy OD stretch, for example, occurs with its companion OD stretch obscured by the much stronger AA feature.Keywords (keywords): dip-infrared spectroscopy; gas phase; hole burning; hydrated electron; supersonic;

Local minima and first-order saddle points on the potential energy surface of the (H2O)6- cluster as described by a recently introduced one-electron model Hamiltonian are located using a combination of the basin-hopping Monte Carlo, doubly-nudged elastic band, and eigenvector-following methods. The minimum energy pathways connecting the transition states and minima are identified and used to construct disconnectivity diagrams. The implications of the calculated rearrangement pathways for experimental studies of (H2O)6- are discussed.Low-energy portion of the disconnectivity diagram of (H2O)6-. Isodensity surfaces for the excess electron are shown for three isomers.

A polarizable force field that explicitly includes contributions from exchange repulsion, dispersion, charge penetration, and multipole electrostatics was developed to describe the interaction between bromine and water. This force field was combined with a polarizable force field for water and used in molecular dynamics simulations to calculate the relative energetics of three bromine clathrate hydrates. The simulations predict the tetragonal structure (Allen, K. W.; Jeffrey, G. A. J. Chem. Phys. 1963, 38, 2304) to be the most stable, with the CS-I and CS-II cubic structures being less stable. Although the CS-II species is not the most stable energetically, we argue that it could be formed under conditions of low bromine concentration, in agreement with recent measurements (Goldschleger, I. U.; Kerenskaya, G.; Janda, K. C.; Apkarian, V. A. J. Phys. Chem. A 2008, 112, 787) that provide evidence for three different bromine hydrate crystal types.

A multistate empirical valence bond (MSEVB) model for protonated water clusters, which incorporates the TIP4P water model, is presented. This model which is designated MSEVB4P represents a significant improvement over the original model of Voth et al. (J. Phys. Chem. B 1998, 102, 5547) which was based on the TIP3P water model and a smaller improvement over the recently introduced MSEVB3 model (J. Phys. Chem. B 2008, 112, 467) which is based on the SPC/Fw (J. Chem. Phys. 2006, 124, 024503) water model.

In the present study we revisit the problem of the interaction of a water molecule with a single graphite sheet. The density fitting-density functional theory-symmetry-adapted perturbation theory (DF-DFT-SAPT; J. Chem. Phys. 2005, 122, 014103) method is used to calculate the individual contributions arising from the interaction of a water molecule with various acenes, including benzene, coronene, and dodecabenzocoronene. These results are combined with calculations of the electrostatic interactions with water and a C216H36 acene to extrapolate to the limit of an infinite graphite sheet, giving a interaction energy of −2.2 kcal/mol for the water−graphite system, with the assumed geometrical structure with one hydrogen atom pointed down toward the ring system. The structure with two hydrogens pointed down is predicted to be more stable, with a net interaction energy of −2.7 kcal/mol.

Parallel tempering Monte Carlo simulations of the water heptamer anion have been performed for temperatures ranging from 42 to 200 K. At low temperatures, a single peak near 250 meV in the electron binding energy distribution is obtained, while at high temperatures a second, weak peak near 450 meV is observed, in good agreement with those observed experimentally. It is further confirmed that the high electron binding energies are due to hydrogen bonding networks with large net dipole moments and, in most cases, also containing a single double-acceptor monomer, while weak electron binding arises from configurations with smaller dipoles.Electron binding energy distributions from PTMC simulations of (H2O)7- at temperatures of 50 and 200 K are presented. Two representative isomers are shown. PRB is the global minimum isomer and PNFA is the most stable isomer with a double acceptor monomer. The isosurfaces containing 85% of the excess electron density, are shown.

Abstract

The ease with which the pH of water is measured obscures the fact that there is presently no clear molecular description for the hydrated proton. The mid-infrared spectrum of bulk aqueous acid, for example, is too diffuse to establish the roles of the putative Eigen (H₃O⁺) and Zundel (H₅O₂⁺) ion cores. To expose the local environment of the excess charge, we report how the vibrational spectrum of protonated water clusters evolves in the size range from 2 to 11 water molecules. Signature bands indicating embedded Eigen or Zundel limiting forms are observed in all of the spectra with the exception of the three- and five-membered clusters. These unique species display bands appearing at intermediate energies, reflecting asymmetric solvation of the core ion. Taken together, the data reveal the pronounced spectral impact of subtle changes in the hydration environment.

•Spurious prediction of non-planar structure of benzene.•Linear dependency with aug-cc-pVTZ basis set.•Need to override default tolerance on eigenvalues of overlap matrix.MP2 calculations with the full aug-cc-pVTZ basis set give a non-planar structure for benzene. Although this non-physical result can be avoided by using the smaller aug-cc-pVDZ basis set or by scaling or deleting selected functions from the aug-cc-pVTZ basis set, such changes to the basis set can result in calculated values of the frequencies of the b2g out-of-plane vibrations that are considerably underestimated. The origin of this behavior is traced to linear dependency problems with the aug-cc-pVDZ and aug-cc-pVTZ basis sets when used for benzene.

The interaction of a water molecule with the (100) surface of MgO as described by cluster models is studied using MP2, coupled MP2 (MP2C) and symmetry–adapted perturbation theory (SAPT) methods. In addition, diffusion Monte Carlo (DMC) results are presented for several slab models as well as for the smallest, 2X2 cluster model. For the 2X2 model it is found that the MP2C, DMC, and CCSD(T) methods give nearly the same potential energy curve for the water–cluster interaction, whereas the potential energy curve from the SAPT calculations differs slightly from those of the other methods. The interaction of the water molecule with the cluster models of the MgO(100) surface is weakened upon expanding the number of layers from one to two and also upon expanding the description of the layers from 2X2 to 4X4 to 6X6. The SAPT calculations reveal that both these expansions of the cluster model are accompanied by reductions in the magnitudes of the induction and dispersion constributions. The best estimate of the energy for binding an isolated water molecule to the surface obtained from the cluster model calculations is in good agreement with that obtained from the DMC calculations using a 2–layer slab model with periodic boundary conditions.

In a previous study (J. Phys. Chem. C, 2009, 113, 10242–10248) we used density functional theory based symmetry–adapted perturbation theory (DFT–SAPT) calculations of water interacting with benzene (C6H6), coronene (C24H12), and circumcoronene (C54H18) to estimate the interaction energy between a water molecule and a graphene sheet. The present study extends this earlier work by use of a more realistic geometry with the water molecule oriented perpendicular to the acene with both hydrogen atoms pointing down. We also include results for an intermediate C48H18 acene. Extrapolation of the water–acene results gives a value of −3.0 ± 0.15 kcal mol−1 for the binding of a water molecule to graphene. Several popular dispersion-corrected DFT methods are applied to the water–acene systems and the resulting interacting energies are compared to results of the DFT–SAPT calculations in order to assess their performance.