Volume 132, Issue 14, 14 April 2010

Gasphase atomic anions lack bound electronic excited states, yet in solution many of these anions exhibit intense absorption bands due to the presence of excited states, referred to as chargetransfertosolvent (CTTS) states that are bound only by the presence of the solvent. CTTS spectra thus serve as delicate probes of solutesolvent interactions, but the fact that they are created by the interactions of a solute with many solvent molecules makes them a challenge to describe theoretically. In this paper, we use mixed quantum/classical molecular dynamics with the twoelectron Fouriergrid (2EFG) electronic structure method presented in the previous paper [W. J. Glover, R. E. Larsen, and B. J. Schwartz, J. Chem. Phys.132, 144101 (2010)] to simulate the CTTS states of a sodium anion in liquid tetrahydrofuran, . Since our 2EFG method is based on configuration interaction with single and double excitations in a grid basis, it allows for an exact treatment of the two valence electrons of the sodium anion. To simulate , we first develop a new electron–THF pseudopotential, and we verify the accuracy of this potential by reproducing the experimental absorptionspectrum of an excess electron in liquid THF with near quantitative accuracy. We also are able to reproduce the CTTS spectrum of and find that the CTTS states of exhibit a Rydberglike progression due to the preexisting longrange solventpolarization around the anion. We also find that the CTTS states are highly mixed with the disjoint electronic states supported by naturally occurring solvent cavities that exist in liquid THF. This mixing explains why the solvated electrons that are ejected following CTTS excitation appear with their equilibrium absorptionspectrum. The mixing of the CTTS and solventcavity states also explains why the recombination of the electron and its geminate partner occurs on slower time scales when photoexciting in the blue compared to in the red of the CTTS band: blue excitation accesses CTTS states whose charge densities lies further from the position of the anion, whereas red excitation accesses CTTS states that lie primarily within the anion’s first solvation shell. Finally, we see that the radial character of the CTTS states near the core matches that of , explaining why the spectrum of this species appears instantly after photoexciting .
 COMMUNICATIONS


Communications: Evidence for the role of fluctuations in the thermodynamics of nanoscale drops and the implications in computations of the surface tension
View Description Hide DescriptionTestarea deformations are used to analyze vaporliquid interfaces of LennardJones particles by molecular dynamics simulation. For planar vaporliquid interfaces the change in free energy is captured by the average of the corresponding change in energy, the leadingorder contribution. This is consistent with the commonly used mechanical (pressuretensor) route for the surface tension. By contrast for liquiddrops, one finds a large secondorder contribution associated with fluctuations in energy. Both the first and secondorder terms make comparable contributions, invalidating the mechanical relation for the surface tension of small drops. The latter is seen to increase above the planar value for drop radii of particle diameters, followed by an apparent weak maximum and slow decay to the planar limit, consistent with a small negative Tolman length.

Communications: Photoinitiated bond dissociation of bromoiodomethane in solution: Comparison of onephoton and twophoton excitations and the formation of iso and iso
View Description Hide DescriptionBroadband UVvisible femtosecond transient absorption spectroscopy was used to monitor the excited state photochemistry of following onephoton excitation at 266 or 271 nm and twophoton excitation at 395 or 405 nm in 2butanol. The results for onephoton excitation agree with earlier studies in acetonitrile, showing clear formation of iso following cleavage of the C–I bond. In contrast, twophoton excitation at 395 nm results in the appearance of a blueshifted photoproduct absorption band assigned to formation of iso following cleavage of the C–Br bond. The results are discussed in the context of prior experimental and theoretical work and the prospects for optical control of bond cleavage.
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 ARTICLES

 Theoretical Methods and Algorithms

First principles multielectron mixed quantum/classical simulations in the condensed phase. I. An efficient Fouriergrid method for solving the manyelectron problem
View Description Hide DescriptionWe introduce an efficient multielectron firstprinciples based electronic structure method, the twoelectron Fouriergrid (2EFG) approach, that is particularly suited for use in mixed quantum/classical simulations of condensedphase systems. The 2EFG method directly solves for the sixdimensional wave function of a twoelectron Hamiltonian in a Fouriergrid representation such that the effects of electron correlation and exchange are treated exactly for both the ground and excited states. Due to the simplicity of a Fouriergrid representation, the 2EFG is readily parallelizable and we discuss its computational implementation in a distributedmemory parallel environment. We show our method is highly efficient, being able to find twoelectron wave functions in on a modern desktop computer for a calculation this is equivalent to full configuration interaction (FCI) in a basis of 17 million Slater determinants. We benchmark the accuracy of the 2EFG by applying it to two electronic structure test problems: the harmonium atom and the sodium dimer. We find that even with a modest grid basis size, our method converges to the analytically exact solutions of harmonium in both the weakly and strongly correlated electron regimes. Our method also reproduces the lowlying potential energy curves of the sodium dimer to a similar level of accuracy as a valence CI calculation, thus demonstrating its applicability to molecular systems. In the following paper [W. J. Glover, R. E. Larsen, and B. J. Schwartz, J. Chem. Phys.132, 144102 (2010)], we use the 2EFG method to explore the nature of the electronic states that comprise the chargetransfertosolvent absorption band of sodium anions in liquid tetrahydrofuran.

First principles multielectron mixed quantum/classical simulations in the condensed phase. II. The chargetransfertosolvent states of sodium anions in liquid tetrahydrofuran
View Description Hide DescriptionGasphase atomic anions lack bound electronic excited states, yet in solution many of these anions exhibit intense absorption bands due to the presence of excited states, referred to as chargetransfertosolvent (CTTS) states that are bound only by the presence of the solvent. CTTS spectra thus serve as delicate probes of solutesolvent interactions, but the fact that they are created by the interactions of a solute with many solvent molecules makes them a challenge to describe theoretically. In this paper, we use mixed quantum/classical molecular dynamics with the twoelectron Fouriergrid (2EFG) electronic structure method presented in the previous paper [W. J. Glover, R. E. Larsen, and B. J. Schwartz, J. Chem. Phys.132, 144101 (2010)] to simulate the CTTS states of a sodium anion in liquid tetrahydrofuran, . Since our 2EFG method is based on configuration interaction with single and double excitations in a grid basis, it allows for an exact treatment of the two valence electrons of the sodium anion. To simulate , we first develop a new electron–THF pseudopotential, and we verify the accuracy of this potential by reproducing the experimental absorptionspectrum of an excess electron in liquid THF with near quantitative accuracy. We also are able to reproduce the CTTS spectrum of and find that the CTTS states of exhibit a Rydberglike progression due to the preexisting longrange solventpolarization around the anion. We also find that the CTTS states are highly mixed with the disjoint electronic states supported by naturally occurring solvent cavities that exist in liquid THF. This mixing explains why the solvated electrons that are ejected following CTTS excitation appear with their equilibrium absorptionspectrum. The mixing of the CTTS and solventcavity states also explains why the recombination of the electron and its geminate partner occurs on slower time scales when photoexciting in the blue compared to in the red of the CTTS band: blue excitation accesses CTTS states whose charge densities lies further from the position of the anion, whereas red excitation accesses CTTS states that lie primarily within the anion’s first solvation shell. Finally, we see that the radial character of the CTTS states near the core matches that of , explaining why the spectrum of this species appears instantly after photoexciting .

Steady shear rheometry of dissipative particle dynamics models of polymer fluids in reverse Poiseuille flow
View Description Hide DescriptionPolymer fluids are modeled with dissipative particle dynamics (DPD) as undiluted beadspring chains and their solutions. The models are assessed by investigating their steady shearrate properties. NonNewtonian viscosity and normal stress coefficients, for shear rates from the lower to the upper Newtonian regimes, are calculated from both plane Couette and plane Poiseuille flows. The latter is realized as reverse Poiseuille flow (RPF) generated from two Poiseuille flows driven by uniform body forces in opposite directions along twohalves of a computational domain. Periodic boundary conditions ensure the RPF wall velocity to be zero without density fluctuations. In overlapping shearrate regimes the RPF properties are confirmed to be in good agreement with those calculated from plane Couette flow with Lees–Edwards periodic boundary conditions (LECs), the standard virtual rheometer for steady shearrate properties. The concentration and the temperature dependence of the properties of the model fluids are shown to satisfy the principles of concentration and temperature superposition commonly employed in the empirical correlation of real polymerfluid properties. The thermodynamic validity of the equation of state is found to be a crucial factor for the achievement of timetemperature superposition. With these models, RPF is demonstrated to be an accurate and convenient virtual rheometer for the acquisition of steady shearrate rheological properties. It complements, confirms, and extends the results obtained with the standard LEC configuration, and it can be used with the output from other particlebased methods, including molecular dynamics, Brownian dynamics, smooth particle hydrodynamics, and the lattice Boltzmann method.

Basis set consistent revision of the S22 test set of noncovalent interaction energies
View Description Hide DescriptionThe S22 test set of interactionenergies for small model complexes [Phys. Chem. Chem. Phys.8, 1985 (2006)] has been very valuable for benchmarking new and existing methods for noncovalent interactions. However, the basis sets utilized to compute the CCSD(T) interactionenergies for some of the dimers are insufficient to obtain converged results. Here we consistently extrapolate all CCSD(T)/complete basis set (CBS) interactionenergies using larger basis sets for the CCSD(T) component of the computation. The revised values, which we designate S22A, represent the most accurate results to date for this set of dimers. The new values appear to be within a few hundredths of of the true CCSD(T)/CBS limit at the given geometries, but the former S22 values are off by as much as compared to the revised values. Because some of the most promising methods for noncovalent interactions are already achieving this level of agreement (or better) compared to the S22 data, more accurate benchmark values would clearly be helpful. The MP2, SCSMP2, SCSCCSD, SCS(MI)MP2, and B2PLYPD methods have been tested against the more accurate benchmark set. The B2PLYPD method outperforms all other methods tested here, with a mean average deviation of only . However, the consistent, slight underestimation of the interactionenergies computed by the SCSCCSD method (an overall mean absolute deviation and mean deviation of 0.24 and , respectively) suggests that the SCSCCSD method has the potential to become even more accurate with a reoptimization of its parameters for noncovalent interactions.

Hilbertspace treatment of incoherent, timeresolved spectroscopy. I. Formalism, a tensorial classification of highorder orientational gratings and generalized MUPPETS “echoes”
View Description Hide DescriptionTimeresolved spectroscopy that uses more than one incoherent excitation, and thus has multiple periods of time evolution, is becoming more common. A recent example is multiple populationperiod transient spectroscopy (MUPPETS), which is implemented as a highorder transient grating. In this paper, a formalism is developed to treat these types of incoherent spectroscopy in a manner that parallels the Liouvillepathway formalism used to treat multidimensional coherent spectroscopy. A Hilbert space of incoherent (population) states is defined and general expressions for transition and timeevolution operators acting on these states are derived from the corresponding quantum operators. This formalism describes incoherent experiments that involve an arbitrary number of temporal dimensions in terms of pathways through the Hilbert space. Each pathway is associated with a multipletime ratecorrelation function. Previous work has shown that these multipletime correlation functions can measure heterogeneity in electronicrelaxation rates. Thus, they are an analog of coherent “echo” experiments, which measure heterogeneity in frequencies. We show that similar “MUPPETS echo” experiments can be done on any incoherent variable. For a dilute molecular solute, the Hilbertspace method leads to a systematic treatment of multidimensional transient gratings. The extension of irreducibletensor methods to the incoherent Hilbert space results in a classification of orientational gratings of arbitrary order. The general methods developed in this paper are applied more specifically to singlephoton, dipole transitions in the following paper.

Hilbertspace treatment of incoherent, timeresolved spectroscopy. II. Pathway description of optical multiple populationperiod transient spectroscopy
View Description Hide DescriptionThis paper applies the general methods developed in the previous paper (Paper I) to the case of onephoton, dipole transitions of a molecular solute. The results generalize transientgrating spectroscopy to an arbitrarily number of dimensions. Transition and detection operators are derived, and their matrix elements are calculated in the complexvalued basis set developed in Paper I. Selection rules make it possible to analyze which incoherent pathways, and thus which correlation functions, contribute to an dimensional multiple populationperiod transient spectroscopy experiment. Irreducibletensor methods allow the amplitudes of the contributing orientationalcorrelation functions to be calculated for arbitrary polarization conditions. A secondrank polarizationtensor is used to describe the polarization of the pair of beams creating or detecting a grating. Several known results for onedimensional experiments are rederived in this formalism to provide examples of its use.

Finitesize scaling study of the vaporliquid critical properties of confined fluids: Crossover from three dimensions to two dimensions
View Description Hide DescriptionWe perform histogramreweighting grand canonical Monte Carlo simulations of the LennardJones fluid confined between two parallel hard walls and determine the vaporliquid critical and coexistence properties in the range of and , where is the wall separation, is the system size and is the characteristic length. By matching the probability distribution of the ordering operator, , to the threedimensional (3D) and twodimensional (2D) Ising universality classes according to the mixedfield finitesize scaling approach, we establish a “phase diagram” in the plane, showing the boundary between four types of behavior: 3D, quasi3D, quasi2D, and 2D. In order to facilitate 2D critical point calculation, we present a fourparameter analytical expression for the 2D Ising universal distribution. We show that the infinitesystemsize critical points obtained by extrapolation from the apparent 3D and 2D critical points have only minor differences with each other. In agreement with recent reports in the literature [Jana et al., J. Chem. Phys.130, 214707 (2009)], we find departure from linearity in the relationship between critical temperature and inverse wall separation, as well as nonmonotonic dependence of the critical density and the liquid density at coexistence upon wall separation. Additional studies of the ST2 model of water show similar behavior, which suggests that these are quite general properties of confined fluids.

Efficient ab initio path integral hybrid Monte Carlo based on the fourthorder Trotter expansion: Application to fluoride ionwater cluster
View Description Hide DescriptionWe propose an efficient path integral hybrid Monte Carlo (PIHMC) method based on fourthorder Trotter expansion. Here, the secondorder effective force is employed to generate short trial trajectories to avoid computationally expensive Hessian matrix, while the final acceptance is judged based on fourthorder effective potential. The computational performance of our PIHMC scheme is compared with that of conventional PIHMC and PIMD methods based on second and fourthorder Trotter expansions. Our method is applied to onthefly ab initio PIHMC calculation of fluoride ionwater complexes, and , at ambient temperature, particularly focusing on the geometrical isotope effect.

Replica exchange simulation method using temperature and solvent viscosity
View Description Hide DescriptionWe propose an efficient and simple method for fast conformational sampling by introducing the solventviscosity as a parameter to the conventional temperature replica exchange molecular dynamics (TREMD) simulation method. The method, named VREMD (V stands for viscosity), uses both low solventviscosity and high temperature to enhance sampling for each replica; therefore it requires fewer replicas than the TREMD method. To reduce the solventviscosity by a factor of in a molecular dynamics simulation, one can simply reduce the mass of solvent molecules by a factor of . This makes the method as simple as the conventional method. Moreover, thermodynamic and conformational properties of structures in replicas are still useful as long as one has sufficiently sampled the Boltzmann ensemble. The advantage of the present method has been demonstrated with the simulations of the trialanine, decaalanine, and a 16residue hairpin peptides. It shows that the method could reduce the number of replicas by a factor of 1.5 to 2 as compared with the TREMD method.

Analytic gradients for Mukherjee’s multireference coupledcluster method using twoconfigurational selfconsistentfield orbitals
View Description Hide DescriptionAnalytic gradients for the statespecific multireference coupledcluster method suggested by Mahapatra et al. [Mol. Phys.94, 157 (1998)] (MkMRCC) are reported within the singles and doubles approximation using twoconfigurational selfconsistent field (TCSCF) orbitals. The present implementation extends our previous work on MkMRCC gradients [E. Prochnow et al., J. Chem. Phys.131, 064109 (2009)] which is based on restricted Hartree–Fock orbitals and consequently the main focus of the present paper is on the treatment of orbital relaxation at the TCSCF level using coupledperturbed TCSCF theory. Geometry optimizations on arynes and nitrenes are presented to illustrate the influence of the orbitals on the computed equilibrium structures. The results are compared to those obtained at the singlereference coupledcluster singles and doubles and at the MkMRCC singles and doubles level of theory when using restricted Hartree–Fock orbitals.

The resolution of the identity approximation for calculations of spinspin contribution to zerofield splitting parameters
View Description Hide DescriptionIn this work, the resolution of the identity (RI) approximation is developed for the calculation of the electronelectron spinspin coupling (SSC) interaction that is a central component of the zerofield splitting (ZFS) term in the effective spin Hamiltonian. The approximated integrals are then used in largescale multireference configuration interaction treatments of the SSC interaction. The SSC contribution to the ZFS is treated using the Breit–Pauli spinspin Hamiltonian in conjunction with firstorder perturbation theory. Test calculations on a set of diatomic molecules reveal that the error of the RI approximation does not exceed even if standard auxiliary basis sets are used. This error of less than 1% is considered to be negligible compared to the presently achievable accuracy of the SSC calculations relative to experimental data. The present development allows the correlated ab initio calculation of ZFS parameters of larger systems such as linear polyenes and linear polyacenes. The basis set convergence of the calculated ZFS values was investigated, and the effect of electronic correlation on the calculated ZFS parameters is discussed.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

binary condensation in a supersonic Laval nozzle: Presence of small clusters inferred from a macroscopic energy balance
View Description Hide DescriptionWe determined the heat released in the condensing flow of a /carrier gas mixture ( for brevity) through a supersonic Laval nozzle by integrating the equations for supersonic flow with condensation, using the static pressure, temperature, and mole fractions of EtOD and monomers [S. Tanimura, B. E. Wyslouzil, M. S. Zahniser, et al., J. Chem. Phys.127, 034305 (2007)] as inputs. By considering the depletion of the monomer species, the deviation of the pressure from the isentropic value, and the heat released, we estimated that of the EtOD molecules are present as pure clusters (dimer to tetramer) upstream of the onset point of condensation. In contrast, clustering was not detected when only pure EtOD was present under the same conditions (temperature and the partial pressure of EtOD) for which clustering was observed in the flow. This suggests that the formation of EtOD clusters is facilitated by in the flow. A comparison of the heat released to the flow and the expected heat of dissociation of the droplets suggests that small EtOD clusters persist downstream of the onset point. Both upstream and downstream of the onset point of condensation, the concentration of these clusters in the nozzle is higher than that expected at equilibrium. A possible mechanism for the overabundance of pure EtOD clusters is that they form in the mixed particles (droplets or clusters) and evaporate from them.

Tracing molecular electronic excitation dynamics in real time and space
View Description Hide DescriptionWe present a method for studying the movement of electrons and energy within and between electronically excited molecules. The dynamically changing state is a manyelectron wavepacket, for which we numerically integrate the Schrödinger equation using the ADC(2) effective Hamiltonian for the particlehole propagator. We develop the tools necessary for following the separate motions of the particles and holes. Total particle and hole densities can be used to give an overview of the dynamics, which can be atomically decomposed in a Mulliken fashion, or individual particle and hole states give a more detailed look at the structure of an excitation. We apply our model to a neon chain, as an illustrative example, projecting an excited eigenstate of an isolated atom onto the coupled system as the initial state. In addition to demonstrating our propagation and analysis machinery, the results show a dramatic difference in excitationenergy transfer rates as a consequence of initial polarization. Furthermore, already in a system with three constituents, an important aspect of multiple coupled systems appears, in that one absorbing system essentially shields another, changing the effective sitewise coupling parameters.

A benchmark study of the vertical electronic spectra of the linear chain radicals and
View Description Hide DescriptionThe ability of coupledcluster models to predict vertical excitation energies is tested on the electronic states of carbonchain radicals of particular relevance to interstellar chemistry. Using spinunrestricted and restricted reference wave functions, the coupledcluster singles and doubles (CCSD) model and a triplesincluding model (CC3) are tested on the radicals and . Both molecules exhibit lowlying excited states with significant doubleexcitation character (as well as states of quartet multiplicity) and are thus challenging cases for excitedstate approaches. In addition, we employ two diagnostics for the reliability of the CC results: the approximate excitation level (AEL) relative to the ground state and the difference between excitation energies obtained with spinunrestricted and spinrestricted reference wave functions (the UR difference). We find that CCSD yields poor excitation energies for states with AEL significantly larger than ca. 1.1 and/or large UR differences, as well as for certain states exhibiting large spin contamination or other inadequacies in the reference determinant. In such cases, connected triple excitations can be included in the model and generally provide improved results. Furthermore, we find that large discrepancies exist between CC and multireference (MR) results for certain states. These disagreements are not related to basisset effects, but likely arise from the lack of spin adaptation in conventional spinorbital CC implementations and active space selection in the MR models.

Quasiclassical dynamics and kinetics of the , atmospheric reactions
View Description Hide DescriptionThe kinetics and dynamics of the title reactions were studied using the quasiclassical trajectory(QCT) method and two ab initio analytical potential energy surfaces (PESs) developed by our group. In addition to the rate constant (: 10–5000 K), we also considered a broad set of dynamic properties as a function of collision energy (up to 1.0 eV) and the rovibrational state of NO . The production of , reaction(1), dominates the reactivity of the system over the conditions studied, as expected from the large energy barriers associated to the exchange reaction,reaction(2). Moreover, the ground PES, which is barrierless for reaction(1), plays a dominant role. Most of the results were interpreted according to the properties of the PESs involved and the kinematics of the system. The QCTrate constants of reaction(1) are in agreement with the experimental data (: 47–3500 K), including very recent low temperature measurements, and also with variational transition state kinetics and most of quantum dynamics calculations. In addition, the QCT average vibrational energy content of the product also agrees with the experimental and quantum data. The PESs used here could also be useful to determine equilibrium and nonequilibrium reaction rates at very high temperatures (e.g., 5000–15 000 K).

Experimental and theoretical study of the absorption properties of thiolated diamondoids
View Description Hide DescriptionNanoscale hybrid systems are a new class of molecular aggregates that offer numerous new possibilities in materials design. Diamondoid thiols are promising nanoscale building blocks for such hybrid systems. They allow the incorporation of functional groups and the investigation of their effects on the unique materials’ properties of diamondoids. Here we combine experimental data with ab initio theory to explore the optical properties of diamondoid thiols and their dependence on size and shape. Agreement between theoretically and experimentally obtained absorption spectra allows the identification of the nature of the optical transitions that are responsible for some photophysical and photochemical processes. We show that the optical properties of diamondoid thiols in the deep UV regime depend on the functionalization site but are largely size independent. Our findings provide an explanation for the disappearance of diamondoid UV photoluminescence upon thiolation for smaller diamondoids. However, our theoretical results indicate that for larger diamondoid thiols beyond the critical size of six diamondoid cages the lowest energy transitions are characterized by diamondoidlike states suggesting that UV luminescence may be regained.
 Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Prewetting transitions of one site associating fluids
View Description Hide DescriptionPrewetting transitions are studied for LennardJones (LJ) based dimer forming associating fluids, on a structureless surface represented by LJ 9–3 type potential, for various association strengths using grandcanonical transition matrix Monte Carlo (GCTMMC) and histogram reweighting techniques. Occurrences of prewetting transition are observed for association strengths: , 4.0, 6.0, 8.0, and 10.0. Structural properties, monomer fraction, and orientation order profile of thinthick film of onesite associating fluids are presented. Wetting temperature, , and prewetting critical temperature, , increases with increasing association strength, which is in agreement with the results of the density functional theory(DFT). Length of prewetting line, on the other hand, is found to decrease first with increasing association energy until and subsequently found to increase substantially for . This behavior is contrary to the prediction from the DFT. We observe that the boundary tension of thinthick film via GCTMMC and finite size scaling exhibits a maximum with respect to association strength.

Positive and negative cluster ions from liquid ethanol by fast ion bombardment
View Description Hide DescriptionSecondary ion mass spectra have been measured for the first time for a liquidethanol target bombarded by 2.0 MeV ions. Positive and negative ion spectra exhibit evidently a series of cluster ions of the forms and , respectively, in addition to light fragment ions from intact parent molecules. It was found that these cluster ions are produced only from liquid phase ethanol. Both positive and negative secondary ion spectra show similar cluster size distributions with almost the same decay slope. We also present for the first time the cluster ion distribution emitted from the liquid at different liquid temperatures.