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Journal of Chemical Physics

Matthew S Church, Timothy J H Hele, Gregory S Ezra, Nandini Ananth
We extend the Mixed Quantum-Classical Initial Value Representation (MQC-IVR), a semiclassical method for computing real-time correlation functions, to electronically nonadiabatic systems using the Meyer-Miller-Stock-Thoss (MMST) Hamiltonian in order to treat electronic and nuclear degrees of freedom (dofs) within a consistent dynamic framework. We introduce an efficient symplectic integration scheme, the MInt algorithm, for numerical time evolution of the phase space variables and monodromy matrix under the non-separable MMST Hamiltonian...
March 14, 2018: Journal of Chemical Physics
Masahiko Machida, Koichiro Kato, Motoyuki Shiga
The isotopologs of liquid water, H2 O, D2 O, and T2 O, are studied systematically by first principles PIMD simulations, in which the whole entity of the electrons and nuclei are treated quantum mechanically. The simulation results are in reasonable agreement with available experimental data on isotope effects, in particular, on the peak shift in the radial distributions of H2 O and D2 O and the shift in the evaporation energies. It is found that, due to differences in nuclear quantum effects, the H atoms in the OH bonds more easily access the dissociative region up to the hydrogen bond center than the D (T) atoms in the OD (OT) bonds...
March 14, 2018: Journal of Chemical Physics
Ankit Raj, Hiro-O Hamaguchi, Henryk A Witek
We report an exhaustive compilation of wavelength-dependent matrix elements over the mean polarizability (α¯) and polarizability anisotropy (γ) operators for the rovibrational states of the H2 , HD, and D2 molecules together with an accompanying computer program for their evaluation. The matrix elements can be readily evaluated using the provided codes for rovibrational states with J = 0-15 and v = 0-4 and for any laser wavelengths in the interval 182.25-1320.6 nm corresponding to popular, commercially available lasers...
March 14, 2018: Journal of Chemical Physics
Pushp Bajaj, Xiao-Gang Wang, Tucker Carrington, Francesco Paesani
Full-dimensional vibrational spectra are calculated for both X- (H2 O) and X- (D2 O) dimers (X = F, Cl, Br, I) at the quantum-mechanical level. The calculations are carried out on two sets of recently developed potential energy functions (PEFs), namely, Thole-type model energy (TTM-nrg) and many-body energy (MB-nrg), using the symmetry-adapted Lanczos algorithm with a product basis set including all six vibrational coordinates. Although both TTM-nrg and MB-nrg PEFs are derived from coupled-cluster single double triple-F12 data obtained in the complete basis set limit, they differ in how many-body effects are represented at short range...
March 14, 2018: Journal of Chemical Physics
Sylvie Antoun, M Ziad Saghir, Seshasai Srinivasan
In multicomponent liquid mixtures, the diffusion flow of chemical species can be induced by temperature gradients, which leads to a separation of the constituent components. This cross effect between temperature and concentration is known as thermodiffusion or the Ludwig-Soret effect. The performance of boundary driven non-equilibrium molecular dynamics along with the enhanced heat exchange (eHEX) algorithm was studied by assessing the thermodiffusion process in n-pentane/n-decane (nC5 -nC10 ) binary mixtures...
March 14, 2018: Journal of Chemical Physics
R Schoonmaker, T Lancaster, S J Clark
Cyclobutadiene has a four-membered carbon ring with two double bonds, but this highly strained molecular configuration is almost square and, via a coordinated motion, the nuclei quantum mechanically tunnels through the high-energy square state to a configuration equivalent to the initial configuration under a 90° rotation. This results in a square ground state, comprising a superposition of two molecular configurations, that is driven by quantum tunneling. Using a quantum mechanical model, and an effective nuclear potential from density functional theory, we calculate the vibrational energy spectrum and the accompanying wavefunctions...
March 14, 2018: Journal of Chemical Physics
Mariana Rossi, Venkat Kapil, Michele Ceriotti
Generalized Langevin Equation (GLE) thermostats have been used very effectively as a tool to manipulate and optimize the sampling of thermodynamic ensembles and the associated static properties. Here we show that a similar, exquisite level of control can be achieved for the dynamical properties computed from thermostatted trajectories. We develop quantitative measures of the disturbance induced by the GLE to the Hamiltonian dynamics of a harmonic oscillator, and show that these analytical results accurately predict the behavior of strongly anharmonic systems...
March 14, 2018: Journal of Chemical Physics
Takashi Kumagai, Janina N Ladenthin, Yair Litman, Mariana Rossi, Leonhard Grill, Sylwester Gawinkowski, Jacek Waluk, Mats Persson
Tautomerization in single porphycene molecules is investigated on Cu(111), Ag(111), and Au(111) surfaces by a combination of low-temperature scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations. It is revealed that the trans configuration is the thermodynamically stable form of porphycene on Cu(111) and Ag(111), whereas the cis configuration occurs as a meta-stable form. The trans → cis or cis → trans conversion on Cu(111) can be induced in an unidirectional fashion by injecting tunneling electrons from the STM tip or heating the surface, respectively...
March 14, 2018: Journal of Chemical Physics
Xuecheng Tao, Philip Shushkov, Thomas F Miller
We describe a path-integral approach for including nuclear quantum effects in non-adiabatic chemical dynamics simulations. For a general physical system with multiple electronic energy levels, a corresponding isomorphic Hamiltonian is introduced such that Boltzmann sampling of the isomorphic Hamiltonian with classical nuclear degrees of freedom yields the exact quantum Boltzmann distribution for the original physical system. In the limit of a single electronic energy level, the isomorphic Hamiltonian reduces to the familiar cases of either ring polymer molecular dynamics (RPMD) or centroid molecular dynamics Hamiltonians, depending on the implementation...
March 14, 2018: Journal of Chemical Physics
Itay Azizi, Yitzhak Rabin
We use Langevin dynamics simulations to study the growth kinetics and the steady-state properties of condensed clusters in a dilute two-dimensional system of particles that are all different (APD) in the sense that each particle is characterized by a randomly chosen interaction parameter. The growth exponents, the transition temperatures, and the steady-state properties of the clusters and of the surrounding gas phase are obtained and compared with those of one-component systems. We investigate the fractionation phenomenon, i...
March 14, 2018: Journal of Chemical Physics
Mark Tuckerman, David Ceperley
Although the observable universe strictly obeys the laws of quantum mechanics, in many instances, a classical description that either ignores quantum effects entirely or accounts for them at a very crude level is sufficient to describe a wide variety of phenomena. However, when this approximation breaks down, as is often the case for processes involving light nuclei, a full quantum treatment becomes indispensable. This Special Topic in The Journal of Chemical Physics showcases recent advances in our understanding of nuclear quantum effects in condensed phases as well as novel algorithmic developments and applications that have enhanced the capability to study these effects...
March 14, 2018: Journal of Chemical Physics
Shuai Zhang, Burkhard Militzer, Lorin X Benedict, François Soubiran, Philip A Sterne, Kevin P Driver
Carbon-hydrogen plasmas and hydrocarbon materials are of broad interest to laser shock experimentalists, high energy density physicists, and astrophysicists. Accurate equations of state (EOSs) of hydrocarbons are valuable for various studies from inertial confinement fusion to planetary science. By combining path integral Monte Carlo (PIMC) results at high temperatures and density functional theory molecular dynamics results at lower temperatures, we compute the EOSs for hydrocarbons from simulations performed at 1473 separate (ρ, T)-points distributed over a range of compositions...
March 14, 2018: Journal of Chemical Physics
Werner Koch, David J Tannor
Stokes phenomenon refers to the fact that an asymptotic expansion of complex functions can differ in different regions of the complex plane, and that beyond the so-called Stokes lines the expansion has an unphysical divergence. An important special case is when the Stokes lines emanate from phase space caustics of a complex trajectory manifold. In this case, symmetry determines that to second order there is a double coverage of the space, one portion of which is unphysical. Building on the seminal but laconic findings of Adachi, we show that the deviation from second order can be used to rigorously determine the Stokes lines and therefore the region of the space that should be removed...
March 14, 2018: Journal of Chemical Physics
Xinzijian Liu, Jian Liu
An exact approach to compute physical properties for general multi-electronic-state (MES) systems in thermal equilibrium is presented. The approach is extended from our recent progress on path integral molecular dynamics (PIMD), Liu et al. [J. Chem. Phys. 145, 024103 (2016)] and Zhang et al. [J. Chem. Phys. 147, 034109 (2017)], for quantum statistical mechanics when a single potential energy surface is involved. We first define an effective potential function that is numerically favorable for MES-PIMD and then derive corresponding estimators in MES-PIMD for evaluating various physical properties...
March 14, 2018: Journal of Chemical Physics
Zelig Chernia, Yoav Tsori
Phase separation in substituted pyridines in water is usually described as an interplay between temperature-driven breakage of hydrogen bonds and the associating interaction of the van der Waals force. In previous quantum-chemical studies, the strength of hydrogen bonding between one water and one pyridine molecules (the 1:1 complex) was assigned a pivotal role. It was accepted that the disassembly of the 1:1 complex at a critical temperature leads to phase separation and formation of the miscibility gap. Yet, for over two decades, notable empirical data and theoretical arguments were presented against that view, thus revealing the need in a revised quantum-mechanical description...
March 14, 2018: Journal of Chemical Physics
Christopher Robertson, Scott Habershon
We present an approximation to the thermal symmetric form of the quantum time-correlation function in the standard position path-integral representation. By transforming to a sum-and-difference position representation and then Taylor-expanding the potential energy surface of the system to second order, the resulting expression provides a harmonic weighting function that approximately recovers the contribution of the phase to the time-correlation function. This method is readily implemented in a Monte Carlo sampling scheme and provides exact results for harmonic potentials (for both linear and non-linear operators) and near-quantitative results for anharmonic systems for low temperatures and times that are likely to be relevant to condensed phase experiments...
March 14, 2018: Journal of Chemical Physics
Giovanni Rillo, Miguel A Morales, David M Ceperley, Carlo Pierleoni
We performed simulations for solid molecular hydrogen at high pressures (250 GPa ≤ P ≤ 500 GPa) along two isotherms at T = 200 K (phase III) and at T = 414 K (phase IV). At T = 200 K, we considered likely candidates for phase III, the C2c and Cmca12 structures, while at T = 414 K in phase IV, we studied the Pc48 structure. We employed both Coupled Electron-Ion Monte Carlo (CEIMC) and Path Integral Molecular Dynamics (PIMD). The latter is based on Density Functional Theory (DFT) with the van der Waals approximation (vdW-DF)...
March 14, 2018: Journal of Chemical Physics
Joseph E Lawrence, David E Manolopoulos
The Wolynes theory of electronically nonadiabatic reaction rates [P. G. Wolynes, J. Chem. Phys. 87, 6559 (1987)] is based on a saddle point approximation to the time integral of a reactive flux autocorrelation function in the nonadiabatic (golden rule) limit. The dominant saddle point is on the imaginary time axis at tsp =iλsp ℏ, and provided λsp lies in the range -β/2≤λsp ≤β/2, it is straightforward to evaluate the rate constant using information obtained from an imaginary time path integral calculation...
March 14, 2018: Journal of Chemical Physics
Parijat Sengupta, Saptarshi Das, Junxia Shi
We study a thermal gradient induced current Ith flow in potassium-doped two-dimensional anisotropic black phosphorus (BP) with semi-Dirac dispersion. The prototype device is a BP channel clamped between two contacts maintained at unequal temperatures. The choice of BP lies in the predicted efficient thermoelectric behaviour. A temperature-induced difference in the Fermi levels of the two contacts drives the current (typified by the electro-thermal conductance) which we calculate using the Landauer transport equation...
March 14, 2018: Journal of Chemical Physics
Igor Poltavsky, Robert A DiStasio, Alexandre Tkatchenko
Nuclear quantum effects (NQE), which include both zero-point motion and tunneling, exhibit quite an impressive range of influence over the equilibrium and dynamical properties of molecules and materials. In this work, we extend our recently proposed perturbed path-integral (PPI) approach for modeling NQE in molecular systems [I. Poltavsky and A. Tkatchenko, Chem. Sci. 7, 1368 (2016)], which successfully combines the advantages of thermodynamic perturbation theory with path-integral molecular dynamics (PIMD), in a number of important directions...
March 14, 2018: Journal of Chemical Physics
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