Research

2004
Hod, O. ; Baer, R. ; Rabani, E. Feasible nanometric magnetoresistance devices. The Journal of Physical Chemistry B 2004, 108, 14807–14810. hod2004.pdf
Kallush, S. ; Band, Y. B. ; Baer, R. Rotational Aspects of short-pulse population transfer in diatomic molecules. Chem. Phys. Lett. 2004, 392, 23.Abstract

A fully-selective population transfer scheme for diatomic molecules using short-duration (

kallush2004.pdf
Jacobi, S. ; Baer, R. The well-tempered auxiliary-field Monte Carlo. The Journal of chemical physics 2004, 120, 43–50.Abstract

The auxiliary-field Monte Carlo (AFMC) is a method for computing ground-state and excited-state energies and other properties of electrons in molecules. For a given basis set, AFMC is an approximation to full-configuration interaction and the accuracy is determined predominantly by an inverse temperature "\beta" parameter. A considerable amount of the dynamical correlation energy is recovered even at small values of \beta. Yet, nondynamical correlation energy is inefficiently treated by AFMC. This is because the statistical error grows with \beta, warranting increasing amount of Monte Carlo sampling. A recently introduced multideterminant variant of AFMC is studied, and the method can be tuned by balancing the sizes of the determinantal space and the \beta parameter with respect to a predefined target accuracy. The well tempered AFMC is considerably more efficient than a naive AFMC. We demonstrate the principles on dissociating hydrogen molecule and torsion of ethylene where we calculate the (unoptimized) torsional barrier and the vertical singlet-triplet

jacobi2004well.pdf
Collepardo-Guevara, R. ; Walter, D. ; Neuhauser, D. ; Baer, R. A Hückel study of the effect of a molecular resonance cavity on the quantum conductance of an alkene wire. Chem. Phys. Lett. 2004, 393, 367–371.Abstract

{We use Huckel theory to examine interference effects on conductance of a wire when a ‘lollypop’ side-chain is bonded to it, acting as a resonance cavity. A clear signature of interference is found at these ballistic conducting systems, stronger in small systems. Gating effects are enhanced by the presence of the loop, where the electronic wavefunctions can experience large changes in phase. Using an ‘interference index’

collepardo-guevara2004.pdf
Baer, R. ; Seideman, T. ; Ilani, S. ; Neuhauser, D. Ab initio study of the alternating current impedance of a molecular junction. J. Chem. Phys. 2004, 120, 3387–3396.Abstract

The small-bias conductance of the C-6 molecule, stretched between two metallic leads, is studied using time-dependent density functional theory within the adiabatic local density approximation. The leads are modeled by jellium slabs, the electronic density and the current density are described on a grid, whereas the core electrons and the highly oscillating valence orbitals are approximated using standard norm-conserving pseudopotentials. The jellium leads are supplemented by a complex absorbing potential that serves to absorb charge reaching the edge of the electrodes and hence mimic irreversible flow into the macroscopic metal. The system is rapidly exposed to a ramp potential directed along the C-6 axis, which gives rise to the onset of charge and current oscillations. As time progresses, a fast redistribution of the molecular charge is observed, which translates into a direct current response. Accompanying the dc signal, alternating current fluctuations of charge and currents within the molecule and the metallic leads are observed. These form the complex impedance of the molecule and are especially strong at the plasmon frequency of the leads and the lowest excitation peak of C-6. We study the molecular conductance in two limits: the strong coupling limit, where the edge atoms of the chain are submerged in the jellium and the weak coupling case, where the carbon atoms and the leads do not overlap spatially. (C) 2004 American Institute of Physics.

baer2004c.pdf
Baer, R. ; Neuhauser, D. ; Weiss, S. Enhanced absorption induced by a metallic nanoshell. Nano Lett. 2004, 4 85–88.Abstract

Nanoshells have been previously shown to have tunable absorption frequencies that are dependent on the ratio of their inner and outer radii. Inspired by this, we ask: can a nanoshell increase the absorption of a small core system embedded within it? A theoretical model is constructed to answer this question. A core, composed of a “jellium” ball of the density of gold is embedded within a jellium nanoshell of nanometric diameter. The shell plasmon frequency is tuned to the core absorption line. A calculation based the time-dependent density functional theory was performed showing a 10 fold increase in core excitation yield.

baer2004a.pdf
Baer, R. ; Neuhauser, D. Real-time linear response for time-dependent density-functional theory. J. Chem. Phys. 2004, 121, 9803–9807.Abstract

We present a linear-response approach for time-dependent density-functional theories using time-adiabatic functionals. The resulting theory can be performed both in the time and in the frequency domain. The derivation considers an impulsive perturbation after which the Kohn-Sham orbitals develop in time autonomously. The equation describing the evolution is not strictly linear in the wave function representation. Only after going into a symplectic real-spinor representation does the linearity make itself explicit. For performing the numerical integration of the resulting equations, yielding the linear response in time, we develop a modified Chebyshev expansion approach. The frequency domain is easily accessible as well by changing the coefficients of the Chebyshev polynomial, yielding the expansion of a formal symplectic Green's operator. (C) 2004 American Institute of Physics.

baer2004b.pdf
Baer, R. ; Siam, N. Real-time study of the adiabatic energy loss in an atomic collision with a metal cluster. The Journal of chemical physics 2004, 121, 6341–6345. baer2004d.pdf
2003
Saravanan, C. ; Shao, Y. ; Baer, R. ; Ross, P. N. ; Head–Gordon, M. Sparse matrix multiplications for linear scaling electronic structure calculations in an atom-centered basis set using multiatom blocks. J. Comput. Chem. 2003, 24, 618–622. saravanan2003sparse.pdf
Lüchow, A. ; Neuhauser, D. ; Ka, J. ; Baer, R. ; Chen, J. ; Mandelshtam, V. A. Computing energy levels by inversion of imaginary-time cross-correlation functions. J. Phys. Chem. A 2003, 107, 7175–7180. luchow2003computing.pdf
Liu, C. ; Walter, D. ; Neuhauser, D. ; Baer, R. Molecular recognition and conductance in crown ethers. J. Am. Chem. Soc. 2003, 125, 13936–13937. liu2003.pdf
Neuhauser, D. ; Baer, R. ; Kosloff, R. Quantum soliton dynamics in vibrational chains: Comparison of fully correlated, mean field, and classical dynamics. J. Chem. Phys. 2003, 118, 5729–5735.Abstract

The dynamics of a chain of vibrational bonds which develop a classical solitary compression wave is simulated. A converged fully correlated quantum mechanical calculation is compared with a time dependent mean field approach (TDSCF) and with a classical simulation. The dynamics were all generated from the same Hamiltonian. The TDSCF and classical calculations show a fully developed solitary wave with the expected dependence of group velocity on amplitude. The full quantum calculations show a solitary-like wave which propagates for a while but then degrades. The robustness of the compression wave depends on the initial preparation. Evidence of partial recurrence of the wave has also been observed. (C) 2003 American Institute of Physics.

neuhauser2003a.pdf
Neuhauser, D. ; Baer, R. A two-grid time-dependent formalism for the Maxwell equation. Journal of Theoretical & Computational Chemistry 2003, 2 537–546. neuhauser2003.pdf
Hod, O. ; Rabani, E. ; Baer, R. Carbon nanotube closed-ring structures. Phys. Rev. B 2003, 67, 195408.Abstract

We study the structure and stability of closed-ring carbon nanotubes using a theoretical model based on the Brenner-Tersoff potential. Many metastable structures can be produced. We focus on two methods of generating such structures. In the first, a ring is formed by geometric folding and is then relaxed into minimum energy using a minimizing algorithm. Short tubes do not stay closed. Yet tubes longer than 18 nm are kinetically stable. The other method starts from a straight carbon nanotube and folds it adiabatically into a closed-ring structure. The two methods give strikingly different structures. The structures of the second method are more stable and exhibit two buckles, independent of the nanotube length. This result is in strict contradiction to an elastic shell model. We analyze the results for the failure of the elastic model.

hod2003.pdf
Baer, R. ; Neuhauser, D. Ab initio electrical conductance of a molecular wire. Int. J. Quantum Chem. 2003, 91, 524–532. baer2003c.pdf
Baer, R. ; Neuhauser, D. ; Zdanska, P. ; Moiseyev, N. Ionization and high-harmonic generation in aligned benzene by intense circularly polarized light. Phys. Rev. A 2003, 68, 043406. baer2003e.pdf
Baer, R. ; Kouri, D. J. ; Baer, M. ; Hoffman, D. K. General Born–Oppenheimer–Huang approach to systems of electrons and nuclei. The Journal of chemical physics 2003, 119, 6998–7002.Abstract

We reconsider the Born-Oppenheimer-Huang treatment of systems of electrons and nuclei for the case of their interaction with time-dependent fields. Initially, we present a framework in which all expressions derived are formally exact since no truncations are introduced. The objective is to explore the general structure of the equations under the most unrestricted conditions, including the possibility that the electronic basis is dependent both on the nuclear coordinates and on time. We then derive an application of the theory applicable to cases of interaction with strong time-dependent fields. The method truncates the electronic basis only after the time-dependent interaction is taken into account in the electronic wave functions. This leads to theory which is similar to a Born-Oppenheimer-type truncation within the interaction picture. (C) 2003 American Institute of Physics.

baer2003general.pdf
Anderson, S. M. ; Neuhauser, D. ; Baer, R. Trajectory-dependent cellularized frozen Gaussians, a new approach for semiclassical dynamics: Theory and application to He–naphtalene eigenvalues. The Journal of chemical physics 2003, 118, 9103–9108.Abstract

A semiclassical cellular method is proposed. Signals generated by semiclassical techniques generally deteriorate over time as trajectories become chaotic. One approach to remedy this problem has been to have each trajectory weighted by an entire cell of nearby trajectories (Filinov transform). But even in this approach the exponential part of the propagator typically becomes large and positive over time. Here the cellularization (Filinov) parameter is subject to constraints which make it time dependent and trajectory dependent. It also depends on dimensionality, so it ends up as a matrix. Physically, the Filinov transform is done differently in different directions associated with the stability matrix for the phase-essentially a more confined integration in directions where the matrix diverges and a wider integration in other directions. This squelches the contribution from any part of a trajectory that becomes excessively chaotic. A trajectory-dependent cellurized frozen Gaussian is applied here within the Herman-Kluk semiclassical approach. It is tested by looking at a single-particle three-dimensional problem, He attached to a rigid immovable naphtalene, where it is shown to be more accurate than the original HK approach, without the divergence of the correlation function common in the usual cellular dynamics (HK) formulation, and is able to separate a low-lying excited state from the ground state. (C) 2003 American Institute of Physics.

anderson2003.pdf
2002
Szichman, H. ; Baer, R. A five-dimensional quantum mechanical study of the H+ CH 4 –> H 2+ CH 3 reaction. The Journal of chemical physics 2002, 117, 7614–7623. szichman2002.pdf
Rabani, E. ; Baer, R. Computational chemistry of quantum mechanical processes - Foreword by the Guest Editors. Isr. J. Chem. 2002, 42, I-I.

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