Volume 136, Issue 22, 14 June 2012

In this paper, we investigate the effect of pressure on the molecular dynamics of protic ionic liquid lidocaine hydrochloride, a commonly used pharmaceutical, by means of dielectric spectroscopy and pressuretemperaturevolume methods. We observed that near T_{g} the pressure dependence of conductivityrelaxation times reveals a peculiar behavior, which can be treated as a manifestation of decoupling between ion migration and structural relaxation times. Moreover, we discuss the validity of thermodynamic scaling in lidocaine HCl. We also employed the temperaturevolume Avramov model to determine the value of pressure coefficient of glass transition temperature, dT _{ g }/dP_{ P = 0.1}. Finally, we investigate the role of thermal and density fluctuations in controlling of molecular dynamics of the examined compound.
 ARTICLES

 Theoretical Methods and Algorithms

Constrained spindensity dynamics of an ironsulfur complex: Ferredoxin cofactor
View Description Hide DescriptionThe computation of antiferromagnetic exchange coupling constants J by means of efficient densitybased approaches requires in practice to take care of both spin projection to approximate the low spin ground state and proper localization of the magnetic orbitals at the transition metal centers. This is demonstrated here by a combined approach where the extended broken–symmetry (EBS) technique is employed to include the former aspect, while spin density constraints are applied to ensure the latter. This constrained EBS (CEBS) approach allows us to carry out ab initiomolecular dynamics on a spinprojected low spin potential energy surface that is generated onthefly by propagating two coupled determinants and thereby accessing the antiferromagnetic coupling along the trajectory. When applied to the prototypical model of the oxidized [2Fe2S] cofactor in Ferredoxins, [Fe_{2}S_{2}(SH)_{4}]^{2−}, at room temperature, CEBS leads to remarkably good results for geometrical structures and coupling constants J.

Rapid anharmonic vibrational corrections derived from partial Hessian analysis
View Description Hide DescriptionVibrational analysis within a partial Hessian framework can successfully describe the vibrational properties of a variety of systems where the vibrational modes of interest are localized within a speciﬁc region of the system. We have developed a new approach to calculating anharmonic frequencies based on vibrational frequencies and normal modes obtained from a partial Hessian analysis using secondorder vibrational perturbation theory and the transition optimized shifted Hermite method. This allows anharmonic frequencies for vibrational modes that are spatially localized to be determined at a significantly reduced computational cost. Several molecular systems are examined in order to demonstrate the effectiveness of this method including organic molecules adsorbed on the Si(100)2×1 surface, model peptides in solution, and the C–H stretching region of polycyclic aromatic hydrocarbons. Overall, for a range of systems, anharmonic frequencies calculated using the partial Hessian approach are found to be in close agreement with the results obtained using full anharmonic calculations while providing a significant reduction in computational cost.

Optimizing hierarchical equations of motion for quantum dissipation and quantifying quantum bath effects on quantum transfer mechanisms
View Description Hide DescriptionWe present an optimized hierarchical equations of motion theory for quantum dissipation in multiple Brownian oscillators bath environment, followed by a mechanistic study on a model donorbridgeacceptor system. We show that the optimal hierarchy construction, via the memoryfrequency decomposition for any specified Brownian oscillators bath, is generally achievable through a universal prescreening search. The algorithm goes by identifying the candidates for the best be just some selected Padé spectrum decomposition based schemes, together with a priori accuracy control criterions on the sole approximation, the whitenoise residue ansatz, involved in the hierarchical construction. Beside the universal screening search, we also analytically identify the best for the case of Drude dissipation and that for the Brownian oscillators environment without strongly underdamped bath vibrations. For the mechanistic study, we quantify the quantum nature of bath influence and further address the issue of localization versus delocalization. Proposed are a reduced system entropymeasure and a stateresolved constructive versus destructive interference measure. Their performances on quantifying the correlated systemenvironment coherence are exemplified in conjunction with the optimized hierarchical equations of motion evaluation of the model system dynamics, at some representing bath parameters and temperatures. Analysis also reveals the localization to delocalization transition as temperature decreases.

Surface hopping simulation of vibrational predissociation of methanol dimer
View Description Hide DescriptionThe mixed quantumclassical surface hopping method is applied to the vibrational predissociation of methanol dimer, and the results are compared to more exact quantum calculations. Utilizing the vibrational SCF basis, the predissociation problem is cast into a curve crossing problem between dissociative and quasibound surfaces with different vibrational character. The varied features of the dissociativesurfaces, arising from the large amplitude OH torsion, generate rich predissociationdynamics. The fewest switches surface hopping algorithm of Tully [J. Chem. Phys.93, 1061 (1990)10.1063/1.459170] is applied to both diabatic and adiabatic representations. The comparison affords new insight into the criterion for selecting the suitable representation. The adiabatic method's difficulty with low energy trajectories is highlighted. In the normal crossing case, the diabatic calculations yield good results, albeit showing its limitation in situations where tunneling is important. The quadratic scaling of the rates on coupling strength is confirmed. An interesting resonance behavior is identified and is dealt with using a simple decoherence scheme. For low lying dissociativesurfaces that do not cross the quasibound surface, the diabatic method tends to overestimate the predissociation rate whereas the adiabatic method is qualitatively correct. Analysis reveals the major culprits involve Rabilike oscillation, treatment of classically forbidden hops, and overcoherence. Improvements of the surface hopping results are achieved by adopting a few changes to the original surface hopping algorithms.

An adaptive potential energy surface generation method using curvilinear valence coordinates
View Description Hide DescriptionAn automatic BornOppenheimer potential energy surface (PES) generation method AGAPES is presented designed for the calculation of vibrational spectra of large rigid and semirigid polyatomic molecules within the midinfrared energy range. An adaptive approach guided by information from intermediate vibrational calculations in connection with a multimode expansion of the PES in internal valence coordinates is used and its versatility is tested for a selection of molecules: HNO, HClCO, and formaldoxime. Significant computational savings are reported. The possibility of linear scaling of the sampling grid size with the molecular size due to decrease of correlation of remote coordinates in large molecules is examined and finally, possible improvements are suggested.

Energy conservation in molecular dynamics simulations of classical systems
View Description Hide DescriptionClassical Newtonian dynamics is analytic and the energy of an isolated system is conserved. The energy of such a system, obtained by the discrete “Verlet” algorithm commonly used in molecular dynamics simulations,fluctuates but is conserved in the mean. This is explained by the existence of a “shadow Hamiltonian” [S. Toxvaerd, Phys. Rev. E50, 2271 (1994)], i.e., a Hamiltonian close to the original H with the property that the discrete positions of the Verlet algorithm for H lie on the analytic trajectories of . The shadow Hamiltonian can be obtained from H by an asymptotic expansion in the time step length. Here we use the first nontrivial term in this expansion to obtain an improved estimate of the discrete values of the energy. The investigation is performed for a representative system with LennardJones pair interactions. The simulations show that inclusion of this term reduces the standard deviation of the energy fluctuations by a factor of 100 for typical values of the time step length. Simulations further show that the energy is conserved for at least one hundred million time steps provided the potential and its first four derivatives are continuous at the cutoff. Finally, we show analytically as well as numerically that energy conservation is not sensitive to roundoff errors.

A simplified implementation of van der Waals density functionals for firstprinciples molecular dynamics applications
View Description Hide DescriptionWe present a simplified implementation of the nonlocal van der Waals correlation functional introduced by Dion et al. [Phys. Rev. Lett.92, 246401 (2004)] and reformulated by RománPérez et al. [Phys. Rev. Lett.103, 096102 (2009)]. The proposed numerical approach removes the logarithmic singularity of the kernel function. Complete expressions of the selfconsistent correlation potential and of the stress tensor are given. Combined with various choices of exchange functionals, five versions of van der Waals density functionals are implemented. Applications to the computation of the interaction energy of the benzenewater complex and to the computation of the equilibrium cell parameters of the benzene crystal are presented. As an example of crystal structure calculation involving a mixture of hydrogen bonding and dispersion interactions, we compute the equilibrium structure of two polymorphs of aspirin (2acetoxybenzoic acid, C_{9}H_{8}O_{4}) in the P2_{1}/c monoclinic structure.

The instantaneous shear modulus in the shoving model
View Description Hide DescriptionWe point out that the instantaneous shear modulusG _{∞} of the shoving model for the nonArrhenius temperature dependence of viscous liquids’ relaxation time is the experimentally accessible highfrequency plateau modulus, not the idealized instantaneous affine shear modulus that cannot be measured. Data for a large selection of metallic glasses are compared to three different versions of the shoving model. The original shearmodulus based version shows a slight correlation to the Poisson ratio, which is eliminated by the energylandscape formulation of the model in which the bulk modulus plays a minor role.

Constraining density functional approximations to yield selfinteraction free potentials
View Description Hide DescriptionSelfinteractions (SIs) are a major problem in density functional approximations and the source of serious divergence from experimental results. Here, we propose to optimize density functional total energies in terms of the effective local potential, under constraints for the effective potential that guarantee it is free from SI errors and consequently asymptotically correct. More specifically, we constrain the Hartree, exchange and correlation potential to be the electrostatic potential of a nonnegative effective repulsive density of N − 1 electrons. In this way, the optimal effective potentials exhibit the correct asymptotic decay, resulting in significantly improved oneelectron properties.

Nuclear chargedistribution effects on the NMR spectroscopy parameters
View Description Hide DescriptionWe present here a systematic study about the influence of the size and type of nuclear chargedistribution models (Gaussian and pointlike) on the NMRspectroscopic parameters, the nuclear magnetic shielding σ and the indirect nuclear spin Jcoupling. We found that relativistic effects largely enhance the nuclear chargedistribution effects (NChDE) on those parameters being them quite sensitive to the nuclear model adopted for calculations. Results for two rare gas atoms (Kr, Rn) and few molecular systems like HX, (X = Br, I, At), CH_{4}, SnH_{4}, SnIH_{3}, SnI_{2}H_{2}, and PbIH_{3} are presented. Jcouplings are more sensitive than shieldings in both, relativistic and nonrelativistic (NR) regimes. The highest effect (close to 11% of variation in relativistic calculations with that two different nuclear models) is observed for J(Pb–I) in PbIH_{3}. A similar effect is found for J(Pb–H) in the same molecule, close to 9%. The NChDE for σ(Sn) in SnI_{4−n }H_{ n } with n = 1, 2 is as large as few ppm (between 3 and 8.56 ppm). For J(Sn–H) in this set of molecules, it goes from 37 Hz for SnH_{4} to 54 Hz for SnI_{2}H_{2}. Furthermore, we found that the vicinal NChDE is very small though not zero. For ^{1}J(Sn–H) in SnIH_{3}, the NChDE of iodine is close to 2 Hz (0.1%). We also studied the NChDE on the ground state electronic energies of atoms and molecules. We found that these effects are only important within the relativistic regime but not within the NR one. They are in good agreement with previous works.

Transcorrelated calculations of homogeneous electron gases
View Description Hide DescriptionWe have constructed the complete transcorrelated equation for homogeneous electron gases and investigated this equation on two and threedimensional systems. Correct asymptotic behaviours of the correlation factors can be easily obtained from the transcorrelated equation, both the longrange RPA type decay and the shortrange spin dependent cusp conditions. The complete transcorrelated equation is solved numerically and the outcome correlation energies agree very well with variational quantum Monte Carlo results. Possible simplifications of the transcorrelated calculations are discussed, where we find that the RPA equation for the correlation factor can be considerably improved by adding one more term in the equation.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Determining the appropriate exchangecorrelation functional for timedependent density functional theory studies of chargetransfer excitations in organic dyes
View Description Hide DescriptionUVVis spectra are calculated using timedependent density functional theory for several organic dyes – 4(N, Ndimethylamino) benzonitrile, alizarin, squaraine, polyenelinker dyes, oligothiophenecontaining coumarin dyes (NKX series) and triphenylaminedonor dyes. Most of these dyes (except, for the first two) or their derivatives are considered to be promising organic dyes for dyesensitized solar cells. An accurate description of the photophysics of such dyes is imperative for understanding and creating better dyes. To this end, we studied the dyes within several approximations to the exchangecorrelation functional. The chosen functionals – PBE, M06L, B3LYP, M06, CAMB3LYP, and wB97 – represent the various classes of approximations that are currently being used to study material properties. From amongst the six approximations studied here, CAMB3LYP outperformed the others in its description of chargetransferexcitations in most (though, not all) of the dyes. This study shows why it is difficult to choose a particular functional a priori, especially when starting out with a new dye for solar cell application. A possible way to judge the fitness of an approximation is used in this work and it is shown to provide a good quantitative guideline for subsequent research in this field.

Fulldimensional (15dimensional) ab initio analytical potential energy surface for the cluster
View Description Hide DescriptionFulldimensional ab initiopotential energy surface is constructed for the cluster. The surface is a fit to roughly 160 000 interaction energies obtained with secondorder MöllerPlesset perturbation theory and the ccpVQZ basis set, using the invariant polynomial method [B. J. Braams and J. M. Bowman, Int. Rev. Phys. Chem.28, 577 (2009)10.1080/01442350903234923]. We employ permutationally invariant basis functions in Morsetype variables for all the internuclear distances to incorporate permutational symmetry with respect to interchange of H atoms into the representation of the surface. We describe how different configurations are selected in order to create the database of the interaction energies for the linear least squares fitting procedure. The rootmeansquare error of the fit is 170 cm^{−1} for the entire data set. The surface dissociates correctly to the + H_{2} fragments. A detailed analysis of its topology, as well as comparison with additional ab initio calculations, including harmonic frequencies, verify the quality and accuracy of the parameterized potential. This is the first attempt to present an analytical representation of the 15dimensional surface of the cluster for carrying out dynamics studies.

Effects of adiabatic, relativistic, and quantum electrodynamics interactions on the pair potential and thermophysical properties of helium
View Description Hide DescriptionThe adiabatic, relativistic, and quantum electrodynamics (QED) contributions to the pair potential of helium were computed, fitted separately, and applied, together with the nonrelativistic BornOppenheimer (BO) potential, in calculations of thermophysical properties of helium and of the properties of the helium dimer. An analysis of the convergence patterns of the calculations with increasing basis set sizes allowed us to estimate the uncertainties of the total interaction energy to be below 50 ppm for interatomic separations R smaller than 4 bohrs and for the distance R = 5.6 bohrs. For other separations, the relative uncertainties are up to an order of magnitude larger (and obviously still larger near R = 4.8 bohrs where the potential crosses zero) and are dominated by the uncertainties of the nonrelativistic BO component. These estimates also include the contributions from the neglected relativistic and QED terms proportional to the fourth and higher powers of the finestructure constant α. To obtain such high accuracy, it was necessary to employ explicitly correlated Gaussian expansions containing up to 2400 terms for smaller R (all R in the case of a QED component) and optimized orbital bases up to the cardinal number X = 7 for larger R. Nearexact asymptotic constants were used to describe the largeR behavior of all components. The fitted potential, exhibiting the minimum of −10.996 ± 0.004 K at R = 5.608 0 ± 0.000 1 bohr, was used to determine properties of the very weakly bound ^{4}He_{2} dimer and thermophysical properties of gaseous helium. It is shown that the CasimirPolder retardation effect, increasing the dimer size by about 2 Å relative to the nonrelativistic BO value, is almost completely accounted for by the inclusion of the Breitinteraction and the ArakiSucher contributions to the potential, of the order α^{2} and α^{3}, respectively. The remaining retardation effect, of the order of α^{4} and higher, is practically negligible for the bound state, but is important for the thermophysical properties of helium. Such properties computed from our potential have uncertainties that are generally significantly smaller (sometimes by nearly two orders of magnitude) than those of the most accurate measurements and can be used to establish new metrology standards based on properties of lowdensity helium.

Local and nonlocal contributions to molecular firstorder hyperpolarizability: A Hirshfeld partitioning analysis
View Description Hide DescriptionBased on firstprinciples calculations, a decomposition scheme is proposed to investigate the molecular sitespecific firstorder hyperpolarizability (β) responses by means of Hirshfeld population analysis and finite field method. For a molecule, its β is decomposed into local and nonlocal contributions of individual atoms or groups. The former describes the response within the atomic sphere, while the latter describes the contributions from interatomic charge transfer. This scheme is then applied to six prototypical donoracceptor (DA) or DπA molecules for which the local and nonlocal hyperpolarizabilities are evaluated based on their MP2 density. Both the local and nonlocal parts exhibit sitespecific characteristics, but vary differently with molecular structures. The local part depends mainly on the atomic attributes such as electronegativity and charge state, as well as its location in the molecule, while the nonlocal part relates to the ability and distance of charge delocalization within the molecule, increasing rapidly with molecular size. The proposed decomposition scheme provides a way to distinguish atomic or group contributions to molecular hyperpolarizabilities, which is useful in the molecular design for organic nonlinear optical materials.

Interpretation of the photoelectron spectra of superalkali species: Na_{3}O and Na_{3}O^{−}
View Description Hide DescriptionRecently measured photoelectron spectra of the Na_{3}O^{−} anion have been interpreted with the aid of ab initio electron propagator calculations. As in the case of the Li_{3}O^{−}, we propose that the photoionization of ground and excited neutral states, in a sequential two photon absorption mechanism, plays a role in the interpretation of the observed spectrum. The lowest vertical electron detachment energy of Na_{3}O^{−} corresponds to a Dyson orbital that is composed chiefly of diffuse Na s functions and connects a D_{3h} singlet anion to an uncharged species with the same point group. Electron binding energies of isomers of the anion with different point groups or multiplicities have been considered. The relative magnitudes of the ionization energies of the neutral Li_{3}O and Na_{3}O species are also discussed. Whereas the most recent experimental data hold that Na_{3}O has the higher ionization energy, this work asserts the opposite trend.

Theoretical investigation of rotationally inelastic collisions of CH_{2}() with helium
View Description Hide DescriptionFollowing our earlier work on collisions of He with the methylene radical in its excited state [L. Ma, M. H. Alexander, and P. J. Dagdigian, J. Chem. Phys.134, 154307 (2011)]10.1063/1.3575200, we investigate here the analogous relaxation of in its ground electronic state. The molecule is treated as semirigid, with fixed bond lengths but a varying bond angle. We use an ab initiopotential energy surface (PES) which is averaged over the CH_{2} bending angle weighted by the square of the bending wave function. The PES for the interaction of He with CH_{2} in the state is considerably less anisotropic than for interaction with the state since the two 2p electrons on the C atom are evenly distributed among the bonding and nonbonding molecular orbitals. We report quantum scattering calculations of statetostate and total removal cross sections as well as total removal rate constants at room temperature. Because of the less pronounced anisotropy, these cross sections and rate constants are considerably smaller than for collisions of with He. Finally, we investigate the dependence of rotational inelasticity on the bending vibrational quantum number.

AsH_{3} ultraviolet photochemistry: An ab initio view
View Description Hide DescriptionMultireference configuration interaction calculations have been carried out for lowlying electronic states of AsH_{3}. Bending potentials for the nine lowest states of AsH_{3} are obtained in C _{3v } symmetry for As–H distances fixed at the ground state equilibrium value of 2.850 a _{0}, as well as for the minimum energy path constrained to R _{1} = R _{2} = R _{3}. The calculated equilibrium geometry and bond energy for the ground state agree very well with the previous experimental and theoretical data. It is shown that the lowest excited singlet state belongs to the ^{1} A _{1} symmetry (in C _{3v }), in contradiction to the previous calculations. This state is characterized by a planar equilibrium geometry. Asymmetric stretch potential energy surface (PES) cuts along the H_{2}As–H recoil coordinate (at R _{1} = R _{2} = 2.850 a _{0}, θ = 123.9° and 90°) for numerous excited states and twodimensional PESs for the and states up to the dissociation limits are obtained for the first time. The transition moments are calculated as well and used together with the PES data for the analysis of possible photodecay channels of arsine in its first absorption band.

Spectroscopic observation of nitrous oxide pentamers
View Description Hide DescriptionTwo new infrared bands in the ν_{1} fundamental region of N_{2}O are observed in a supersonic jet expansion and assigned to nitrous oxide pentamers. Each band is measured using both ^{14}N_{2} ^{16}O and ^{15}N_{2} ^{16}O. Although they are similar in appearance, the bands have slightly different lower state rotational parameters, and are thus assigned to distinct structural isomers of the pentamer. Cluster calculations using two N_{2}O intermolecular potentials give results in good agreement with the observed spectra, and indicate that the two isomers probably have the same basic structure (which is unsymmetrical), but differ in the alignment (NNO or ONN) of one or two of the constituent monomers. Calculations using a resonant dipole interaction model also support the proposed assignment and structure. These are the first reported highresolution spectra for N_{2}O pentamers.

A pathintegral Langevin equation treatment of lowtemperature doped helium clusters
View Description Hide DescriptionWe present an implementation of path integral molecular dynamics for sampling low temperature properties of doped helium clusters using Langevin dynamics. The robustness of the path integral Langevin equation and whitenoise Langevin equation[M. Ceriotti, M. Parrinello, T. E. Markland, and D. E. Manolopoulos, J. Chem. Phys.133, 124104 (2010)]10.1063/1.3489925 sampling methods are considered for those weakly bound systems with comparison to path integral Monte Carlo (PIMC) in terms of efficiency and accuracy. Using these techniques, convergence studies are performed to confirm the systematic error reduction introduced by increasing the number of discretization steps of the path integral. We comment on the structural and energetic evolution of He_{ N }−CO_{2} clusters from N = 1 to 20. To quantify the importance of both rotations and exchange in our simulations, we present a chemical potential and calculated band origin shifts as a function of cluster size utilizing PIMC sampling that includes these effects. This work also serves to showcase the implementation of path integral simulation techniques within the molecular modelling toolkit [K. Hinsen, J. Comp. Chem.21, 79 (2000)]10.1002/(SICI)1096987X(20000130)21:2<79::AIDJCC1>3.0.CO;2B, an opensource molecular simulation package.