Dipole induced photodetachment dynamics of halide anions

In this article the dynamics of photodetachment from closed shell anion in the presence of two color (bichromatic) laser field have been explored in the context of polar environmental situation. The electronic states of halide ions are modeled by a one dimensional Hamiltonian with a potential V(x) = ?V₀e. The two parameters V₀ and σ are fixed by requiring V(x) to reproduce the experimentally observed ground state ionization energy of the halide ion concerned. The potential so generated are shown to support only one bound state. The time‐dependent Fourier grid Hamiltonian method is used to follow the detachment dynamics with fairly high intensities of light. The environmental effects on the dynamics are sought to be modeled by two ways i) allowing the well depth (V₀) to fluctuate randomly V₀(t) = V₀[1+ΔVR(t)]; R(t) randomly fluctuates between +1 and ?1 with time, when ΔV (strength of fluctuation) is fixed and ii) in the presence of perturbation produced by a neighboring solvent dipole, which changes the effective potential. The detachment rate constant is studied as a function of important system parameters of the used light field. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Modeling quantum dynamics of photodetachment from closed‐shell anions: Static versus fluctuating well‐depth models

The electronic states of halide ions are modeled by a one‐dimensional Hamiltonian with a potential V(x)=−V₀e. The two parameters V₀ and σ are fixed by requiring V(x) to reproduce the experimentally observed ground‐state ionization potentials of the halide ions concerned. The potentials so generated are shown to support only one bound state in each case. The time‐dependent Fourier grid Hamiltonian method is used to follow the ionization dynamics in monochromatic light of fairly high intensities. The total Hamiltonian, in the most general case, reads H(t)=P/2m−V₀e−ϵ₀s(t)ex sin(ωt). For pulsed fields [s(t)=sin²(πt/t_p)], the computed ionization rate constants are seen to increase with increase in the peak intensity (ϵ₀) of the electric field of light. The possibility of additional transient bound states being generated at the high intensities of light and its possible consequences on the observed ionization rates are explored. The environmental effects on the dynamics are sought to be modeled by allowing the well depth (V₀) to fluctuate randomly [V₀(t)=V₀+ΔVR(t); R(t) randomly fluctuates between +1 and −1 with time, ΔV is fixed]. The ionization rate constants (k_ϵ) are shown to be significantly affected by fluctuations in V₀ and pass through a well‐defined minimum in each case for a certain specified frequency of fluctuation. An alternative model potential V(x)=−V₀e^−σx is also shown to yield similar results. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 469–478, 1999     

Effects of thermal modulation on tunneling through a fluctuating barrier in presence of bichromatic electromagnetic field

The effects of thermal modulation on the tunneling dynamics of a particle moving in a bistable potential with a fluctuating barrier in the presence of bichromatic electromagnetic field are examined. This study brings to light various interesting features. While rate constant increases steadily with increasing fluctuation strength, a stochastic resonance‐like behavior is exhibited when tunneling rate is plotted as a function of barrier fluctuation frequency. The tunneling rate constant decreases with the increase in the differences of the strength of the two electric fields; on the other hand, it displays a maxima when plotted as a function of the ratio of the frequencies of the two fields. When the rate constant is plotted as a function of temperature, it increases and eventually settles down to a saturation value. We have also studied the effect of the phase difference between the two electric fields on the tunneling behavior in the presence of thermal modulation. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Tunneling through a fluctuating barrier in the presence of a periodically driving field

The zero and finite temperature tunneling dynamics of a periodically driven particle moving in a bistable potential with a fluctuating barrier is studied. We have focused on the influence of barrier fluctuation and thermal modulation on the tunneling processes in the presence of a driving field. At zero temperature, for a fixed strength of the driving field, both the tunneling probability and rate passes through a well-defined minimum when plotted as a function of fluctuation frequency while it reveals a clear maximum as a function of driving frequency. However, at T > 0 the tunneling probability and rate show two maxima as a function of both fluctuation frequency and driving frequency. In both zero and finite temperature, the tunneling rate constant decreases with increasing fluctuation strength. So, the barrier fluctuation may enhance the stability of a periodically driven system. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Tunneling dynamics of a double‐well oscillator: Effects of barrier fluctuation and thermal modulation

The tunneling dynamics of a particle moving in a bistable potential with fluctuating barrier is studied. For barriers fluctuating randomly in time we show by exact numerical calculation the significant effect of barrier fluctuation on the tunneling behavior of the particle. At nonzero temperatures the computed tunneling rate constant passes through a maximum when plotted against fluctuation frequency. The resonant frequency (at which the maximum appears) slowly decreases with increase in temperature and attains a constant value at higher temperature and it increases linearly with increase in barrier height of the potential. Another important observation is that in presence of barrier fluctuation the dependence of tunneling rate constant on temperature is strongly guided by the barrier fluctuation frequency. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 280–285, 2003     

Reactivity dynamics of a confined molecule in presence of an external magnetic field

Reactivity dynamics and stability of a confined hydrogen molecule in presence of an external magnetic field has been studied using quantum fluid density functional theory. Dynamic profiles of various reactivity parameters such as hardness, electrophilicity, magnetizability, phase volume, entropy, etc. have been studied within a confined environment. Responses in the reactivity parameters as well as the associated electronic structure principles validate the stability of the confined H₂ molecule in ground and excited states in presence of an external magnetic field. Confinement to the system has been imposed by the Dirichlet type boundary condition. Confinement and excitation act in opposite directions. Ground state type dynamics is obtained on simultaneous electronic excitation and confinement. © 2014 Wiley Periodicals, Inc.     

On-the-Fly Ring-Polymer Molecular Dynamics Calculations of the Dissociative Photodetachment Process of the Oxalate Anion

The dissociative photodetachment dynamics of the oxalate anion, C₂O₄H⁻ + hν → CO₂ + HOCO + e⁻, were theoretically studied using the on-the-fly path-integral and ring-polymer molecular dynamics methods, which can account for nuclear quantum effects at the density-functional theory level in order to compare with the recent experimental study using photoelectron–photofragment coincidence spectroscopy. To reduce computational time, the force acting on each bead of ring-polymer was approximately calculated from the first and second derivatives of the potential energy at the centroid position of the nuclei beads. We find that the calculated photoelectron spectrum qualitatively reproduces the experimental spectrum and that nuclear quantum effects are playing a role in determining spectral widths. The calculated coincidence spectrum is found to reasonably reproduce the experimental spectrum, indicating that a relatively large energy is partitioned into the relative kinetic energy between the CO₂ and HOCO fragments. This is because photodetachment of the parent anion leads to Franck–Condon transition to the repulsive region of the neutral potential energy surface. We also find that the dissociation dynamics are slightly different between the two isomers of the C₂O₄H⁻ anion with closed- and open-form structures.     

Chemiluminescence of 1,2‐dioxetanone studied by a closed‐shell DFT approach

The chemiluminescence of simple 1,2‐dioxetanone has already been studied by both multiconfigurational and density functional theory calculations. The former approach revealed a step‐wise biradical mechanism for its decomposition, whereas the latter revealed a concerted mechanism. The first approach was not in line with both computational and experimental findings, whereas the second mechanism was. Due to these apparent mechanistic contradictions and some concerns regarding our concerted mechanism, and the use of a closed‐shell approach and different methods for geometry and single‐point calculations, we have revisited the chemiluminescence of this molecule. Once again the concerted mechanism was found to be prevailing, and a closed‐shell approach was able to rationalize the chemiluminescence of 1,2‐dioxetanone. It was once again noted that an open‐shell and a step‐wise biradical mechanism cannot explain the chemiluminescence of this molecule. © 2013 Wiley Periodicals, Inc.     

Dissociation of a diatomic molecule induced by discontinuous reversals of a static electric field

A diatomic molecule modeled by a Morse oscillator is shown to undergo dissociation if the external electric field, in which the molecule is placed, undergoes discontinuous reversals. A threshold frequency of reversal is found to exist. The classical behavior is compared with its quantum counterpart. The mechanism of dissociation is analyzed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Atomistic hybrid particle‐field molecular dynamics combined with slip‐springs: Restoring entangled dynamics to simulations of polymer melts

In hybrid particle‐field (hPF) simulations (J. Chem. Phys., 2009 130, 214106), the entangled dynamics of polymer melts is lost due to chain crossability. Chains cross, because the field‐treatment of the nonbonded interactions makes them effectively soft‐core. We introduce a multi‐chain slip‐spring model (J. Chem. Phys., 2013 138, 104907) into the hPF scheme to mimic the topological constraints of entanglements. The structure of the polymer chains is consistent with that of regular molecular dynamics simulations and is not affected by the introduction of slip‐springs. Although slight deviations are seen at short times, dynamical properties such as mean‐square displacements and reorientational relaxation times are in good agreement with traditional molecular dynamics simulations and theoretical predictions at long times.     

Binding structures of tri‐N‐acetyl‐β‐glucosamine in hen egg white lysozyme using molecular dynamics with a polarizable force field

Lysozyme is a well‐studied enzyme that hydrolyzes the β‐(1,4)‐glycosidic linkage of N‐acetyl‐β‐glucosamine (NAG)_n oligomers. The active site of hen egg‐white lysozyme (HEWL) is believed to consist of six subsites, A‐F that can accommodate six sugar residues. We present studies exploring the use of polarizable force fields in conjunction with all‐atom molecular dynamics (MD) simulations to analyze binding structures of complexes of lysozyme and NAG trisaccharide, (NAG)₃. MD trajectories are applied to analyze structures and conformation of the complex as well as protein–ligand interactions, including the hydrogen‐bonding network in the binding pocket. Two binding modes (ABC and BCD) of (NAG)₃ are investigated independently based on a fixed‐charge model and a polarizable model. We also apply molecular mechanics with generalized born and surface area (MM‐GBSA) methods based on MD using both nonpolarizable and polarizable force fields to compute binding free energies. We also study the correlation between root‐mean‐squared deviation and binding free energies of the wildtype and W62Y mutant; we find that for this prototypical system, approaches using the MD trajectories coupled with implicit solvent models are equivalent for polarizable and fixed‐charge models. © 2012 Wiley Periodicals, Inc.     

Exact solution of the relativistic dynamics of a spin‐½ particle moving in a homogeneous magnetic field

The relativistic dynamics of one spin‐½ particle moving in a uniform magnetic field is described by the Hamiltonian $\mathbf{h}^{0}_{D}(\pi)=c\alpha\cdot\pi+\beta mc^{2}$. The discrete (and semidiscrete) eigenvalues and the corresponding eigenspinors are in principle known from the work of Dirac, Rabi, and Bloch. These are extensively reviewed here. Next, exact solutions are worked out for the recoil dynamics in relative coordinates, which involves the Hamiltonian $\mathbf{h}^{0}_{D}(-\mathbf{k})=-c\alpha\cdot\mathbf{k}+\beta mc^{2}$. Exact solutions are also explicitly calculated in the case where the spin‐½ particle has an anomalous magnetic moment such that its Hamiltonian is given by $\mathbf{h}_{D}(\pi)=\mathbf{h}^{0}_{D}(\pi)-\beta\mu_{\mathrm{ano}}\sigma\cdot\mathbf{B}$. Similar exact solutions are derived here when the recoiling particle has an anomalous magnetic moment, that is, the eigenvalues and eigenspinors of the Hamiltonian $\mathbf{h}_{D}(-\mathbf{k})=\mathbf{h}^{0}_{D}(-\mathbf{k})-\beta\mu_{\mathrm{ano}}\sigma\cdot\mathbf{B}$ are explicitly obtained. The diagonalized and separable form of the Hamiltonian h _D(π), written as $\tilde{\mathbf{h}}_{D}(\pi)$, has exceedingly simple forms of eigenspinors. Similarly, the diagonalized and separable form of the operator h _D(? k ), written as $\tilde{\mathbf{h}}_{D}(-\mathbf{k})$, has very simple eigenspinors. The importance of these exact solutions is that the eigenspinors can be used as bases in a calculation involving many spin‐½ particles placed in a uniform magnetic field. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 209–217, 2001     

Nature of closed‐ and open‐shell interactions between noble metals and rare gas atoms

Interactions between noble metals and rare gases have become an interesting topic over the last few years. In this work, a computational study of the open‐shell (d¹⁰s¹) and closed‐shell (d¹⁰s and d¹⁰s²) noble metals (M = Cu, Ag, and Au) with three heaviest rare gas atoms (Rg = Kr, Xe, and Rn) has been performed. Potential energy curves based on ab initio [MP2, MP4, QCISD, and CCSD(T)] and DFT functionals (M06‐2X and CAM‐B3LYP) were obtained for ionic and neutral AuXe complexes. Dissociation energies indicate that neutral metals have the lowest and cationic metals have the highest affinities for interaction with rare gas atoms. For the same metals, there is a continuous increase in dissociation energies (D_e) from Kr to Rn. The nature of bonding and the trend of D_e and equilibrium bond lengths (R_e) have been interpreted by means of quantum theory of atoms in molecules, natural bond orbital, and energy decomposition analysis. © 2013 Wiley Periodicals, Inc.     

Prompt and Slow Electron‐Detachment‐Dissociation/Electron‐Photodetachment‐Dissociation of a 21‐Mer Peptide

Electron detachment dissociation (EDD) and electron photodetachment dissociation (EPD) are relatively new dissociation methods that involve electron detachment followed by radical‐driven dissociation from multiply deprotonated species. EDD yields prompt dissociation whereas only electron detachment is obtained by EPD; subsequent vibrational activation of the charge‐reduced radical anion is required to obtain the product ions. Herein, the fragmentation patterns that were obtained by EDD and by vibrational activation of the charge‐reduced radical anions that were produced through EDD or EPD (activated‐EDD and activated‐EPD) were compared. The observed differences were related to the dissociation kinetics and/or the contribution of electron‐induced dissociation (EID). Time‐resolved double‐resonance experiments were performed to measure the dissociation rate constants of the EDD product ions. Differences in the formation kinetics were revealed between the classical EDD/EPD ′a^._i/′′x_j complementary ions and some ′a^._i/c_i/′′′z^._j product ions, which were produced with slower dissociation rate constants, owing to the presence of specific neighbouring side chains. A new fragmentation pathway is proposed for the formation of the slow‐kinetics ′a^._i ions.     

Hybrid particle‐field molecular dynamics simulations: Parallelization and benchmarks

The parallel implementation of a recently developed hybrid scheme for molecular dynamics (MD) simulations (Milano and Kawakatsu, J Chem Phys 2009, 130, 214106) where self‐consistent field theory (SCF) and particle models are combined is described. Because of the peculiar formulation of the hybrid method, considering single particles interacting with density fields, the most computationally expensive part of the hybrid particle‐field MD simulation can be efficiently parallelized using a straightforward particle decomposition algorithm. Benchmarks of simulations, including comparisons of serial MD and MD‐SCF program profiles, serial MD‐SCF and parallel MD‐SCF program profiles, and parallel benchmarks compared with efficient MD program GROMACS 4.5.4 are tested and reported. The results of benchmarks indicate that the proposed parallelization scheme is very efficient and opens the way to molecular simulations of large scale systems with reasonable computational costs. © 2012 Wiley Periodicals, Inc.     

Design of a reaction field using a linear‐combination‐based isotropic periodic sum method

In our previous study (Takahashi et al., J. Chem. Theory Comput. 2012, 8, 4503), we developed the linear‐combination‐based isotropic periodic sum (LIPS) method. The LIPS method is based on the extended isotropic periodic sum theory that produces a ubiquitous interaction potential function to estimate homogeneous and heterogeneous systems. The LIPS theory also provides the procedure to design a periodic reaction field. To demonstrate this, in the present work, a novel reaction field of the LIPS method was developed. The novel reaction field was labeled LIPS‐SW, because it provides an interaction potential function with a shape that resembles that of the switch function method. To evaluate the ability of the LIPS‐SW method to describe in homogeneous and heterogeneous systems, we carried out molecular dynamics (MD) simulations of bulk water and water–vapor interfacial systems using the LIPS‐SW method. The results of these simulations show that the LIPS‐SW method gives higher accuracy than the conventional interaction potential function of the LIPS method. The accuracy of simulating water–vapor interfacial systems was greatly improved, while that of bulk water systems was maintained using the LIPS‐SW method. We conclude that the LIPS‐SW method shows great potential for high‐accuracy, high‐performance computing to allow large scale MD simulations. © 2014 Wiley Periodicals, Inc.     

Acceleration of self‐consistent field convergence in ab initio molecular dynamics simulation with multiconfigurational wave function

The Lagrange interpolation of molecular orbital (LIMO) method, which reduces the number of self‐consistent field iterations in ab initio molecular dynamics simulations with the Hartree–Fock method and the Kohn–Sham density functional theories, is extended to the theory of multiconfigurational wave functions. We examine two types of treatments for the active orbitals that are partially occupied. The first treatment, as denoted by LIMO(C), is a simple application of the conventional LIMO method to the union of the inactive core and the active orbitals. The second, as denoted by LIMO(S), separately treats the inactive core and the active orbitals. Numerical tests to compare the two treatments clarify that LIMO(S) is superior to LIMO(C). Further applications of LIMO(S) to various systems demonstrate its effectiveness and robustness. © 2014 Wiley Periodicals, Inc.