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PubMed2006

Structures and bonding nature of small monoligated copper clusters (HCN-Cun, n = 1-3) through high-resolution infrared spectroscopy and theory.

The structures, C-H stretching frequencies, and dipole moments of HCN-Cun (n = 1-3) clusters are determined through high-resolution infrared spectroscopy. The complexes are formed and probed within superfluid helium droplets, whereby the helium droplet beam is passed over a resistively heated crucible containing copper shot and then through a gas HCN pickup cell. All complexes are found to be bound to the nitrogen end of the HCN molecule and on the "atop site" of the copper cluster. Through the experimental C-H vibrational shifts of HCN-Cun and ab initio calculations, it was found that the HCN-metal interaction changes from a strong van der Waals bond in n = 1 to a partially covalent bond in HCN-Cu3. Comparisons with existing infrared data on copper surfaces show that the HCN-Cun bond must begin to weaken at very large copper cluster sizes, eventually returning to a van der Waals bond in the bulk copper surface case.

Computer Simulation↗
PubMed2006

Conical intersections in thymine.

The mechanisms which are responsible for the radiationless deactivation of the npi* and pipi* excited singlet states of thymine have been investigated with multireference ab initio methods (the complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory with respect to the CASSCF reference (CASPT2)) as well as with the CC2 (approximated singles and doubles coupled-cluster) method. The vertical excitation energies, the equilibrium geometries of the 1npi*and 1pipi* states, as well as their adiabatic excitation energies have been determined. Three conical intersections of the S1 and S0 energy surfaces have been located. The energy profiles of the excited states and the ground state have been calculated with the CASSCF method along straight-line reaction paths leading from the ground-state equilibrium geometry to the conical intersections. All three conical intersections are characterized by strongly out-of-plane distorted geometries. The lowest-energy conical intersection (CI1) arises from a crossing of the lowest 1pipi* state with the electronic ground state. It is found to be accessible in a barrierless manner from the minimum of the 1pipi* state, providing a direct and fast pathway for the quenching of the population of the lowest optically allowed excited states of thymine. This result explains the complete diffuseness of the absorption spectrum of thymine in supersonic jets. The lowest vibronic levels of the optically nearly dark 1npi* state are predicted to lie below CI1, explaining the experimental observation of a long-lived population of dark excited states in gas-phase thymine.

Computer Simulation↗
PubMed2005

Calculation of adiabatic polarization of atomic photofragments under the influence of long range quadrupole-quadrupole interactions.

General equations for diagonalization of the quadrupole-quadrupole interaction matrix for diatomic molecules are presented. Eigenvalue and eigenvector solutions are tabulated for atoms with levels J < or = 2. The use of the eigenvector solutions for determination of adiabatic molecule-frame photofragment polarization is illustrated, and polarization parameters a and a are tabulated. Even if knowledge of the photofragment scattering matrix is limited, we illustrate how coherent polarization parameters a0(1) and a0(2) can be calculated from the tabulated adiabatic expansion coefficients, for example, by making use of available experimental data.

Algorithms↗
PubMed2006

Vibrational dynamics of the hydrogen bond in H(2)S-HF: Fourier-transform-infrared spectra and ab initio theory.

The rotationally resolved infrared spectrum of the hydrogen bonded complex H(2)S-HF and of its isotopomer D(2)S-DF in the HF/DF stretching range have been observed in a supersonic jet Fourier-transform infrared (FTIR) experiment and indicate a predissociation lifetime of 130 ps for H(2)S-HF. Complementary spectra taken at a temperature of 190 K in a cell without resolved rotational structure indicate the presence of strong anharmonic couplings between low frequency intermolecular modes and the HF donor stretch mode previously observed in other complexes with heavier acceptor molecules without rotational fine structure. The anharmonic analysis of the hot band progressions and of the rotational data confirm the coupling mechanism. The coupling constants and the absolute frequency of the hydrogen bonded stretch mode are in excellent agreement with theoretical predictions based on adiabatic variational calculations on potential surfaces computed at MP2 and CCSD(T) level. Complementary calculations with a perturbational approach further confirm the coupling model.

Computer Simulation↗
PubMed2006

A simple algebraic derivation of the Obara-Saika scheme for general two-electron interaction potentials.

A new derivation is presented for the recursion relation of Obara and Saika (OS) for two-electron integrals over Gaussian basis functions for general interaction potentials g(r(12)), where r(12) denotes the interelectronic distance. The decisive vertical OS recursion is proved directly from the recursion relation for Gaussian basis functions and the structure of the primitive integral expression for s functions. The resulting simple formulae greatly facilitate extensions of OS-based codes for Coulomb interactions to general g, which has already proved useful in implementations. The present derivation further extends the validity of the OS recursion beyond interactions covered so far.

Algorithms↗
PubMed2004

Extrapolating bound state data of anions into the metastable domain.

Computing energies of electronically metastable resonance states is still a great challenge. Both scattering techniques and quantum chemistry based L2 methods are very time consuming. Here we investigate two more economical extrapolation methods. Extrapolating bound states energies into the metastable region using increased nuclear charges has been suggested almost 20 years ago. We critically evaluate this attractive technique employing our complex absorbing potential/Green's function method that allows us to follow a bound state into the continuum. Using the (2)Pi(g) resonance of N2- and the (2)Pi(u) resonance of CO2- as examples, we found that the extrapolation works suprisingly well. The second extrapolation method involves increasing of bond lengths until the sought resonance becomes stable. The keystone is to extrapolate the attachment energy and not the total energy of the system. This method has the great advantage that the whole potential energy curve is obtained with quite good accuracy by the extrapolation. Limitations of the two techniques are discussed.

Journal Article↗
PubMed2004

Absolute electron-impact total ionization cross sections of chlorofluoromethanes.

An experimental study is reported on the electron-impact total ionization cross sections (TICSs) of CCl4, CCl3F, CCl2F2, and CClF3 molecules. The kinetic energy of the colliding electrons was in the 10-85 eV range. TICSs were obtained as the sum of the partial ionization cross sections of all fragment ions, measured and identified in a linear double focusing time-of-flight mass spectrometer. The resulting TICS profiles--as a function of the electron-impact energy--have been compared both with those computed by ab initio and (semi)empirical methods and with the available experimental data. The computational methods used include the binary-encounter-Bethe (BEB) modified to include atoms with principal quantum numbers n> or =3, the Deutsch and Märk (DM) formalism, and the modified additivity rule (MAR). It is concluded that both modified BEB and DM methods fit the experimental TICS for (CF4), CClF3, CCl2F2, CCl3F, and CCl4 to a high accuracy, in contrast with the poor accord of the MAR method. A discussion on the factors influencing the discrepancies of the fittings is presented.

Journal Article↗
PubMed2005

Bose-Einstein condensates in the presence of a magnetic trap and optical lattice.

In this paper we consider solutions of a nonlinear Schrodinger equation with a parabolic and a periodic potential motivated from the dynamics of Bose-Einstein condensates. Our starting point is the corresponding linear problem which we analyze through regular perturbation and homogenization techniques. We then use Lyapunov-Schmidt theory to establish the persistence and bifurcation of the linear states in the presence of attractive and repulsive nonlinear inter-particle interactions. Stability of such solutions is also examined and a count is given of the potential real, complex and imaginary eigenvalues with negative Krein signature that such solutions may possess. The results are corroborated with numerical computations.

Algorithms↗
PubMed2006

Accurate potential energy surface and quantum reaction rate calculations for the H+CH4-->H2+CH3 reaction.

Calculations for the cumulative reaction probability N(E) (for J=0) and the thermal rate constant k(T) of the H+CH(4)-->H(2)+CH(3) reaction are presented. Accurate electronic structure calculations and a converged Shepard-interpolation approach are used to construct a potential energy surface which is specifically designed to allow the precise calculation of k(T) and N(E). Accurate quantum dynamics calculations employing flux correlation functions and multiconfigurational time-dependent Hartree wave packet propagation compute N(E) and k(T) based on this potential energy surface. The present work describes in detail the various convergence test performed to investigate the accuracy of the calculations at each step. These tests demonstrate the predictive power of the present calculations. In addition, approximate approaches for reaction rate calculations are discussed. A quite accurate approximation can be obtained from a potential energy surface which includes only interpolation points on the minimum energy path.

Journal Article↗
PubMed2006

Computation of dipole, quadrupole, and octupole surfaces from the variational two-electron reduced density matrix method.

Recent advances in the direct determination of the two-electron reduced density matrix (2-RDM) by imposing known N-representability conditions have mostly focused on the accuracy of molecular potential energy surfaces where multireference effects are significant. While the norm of the 2-RDM's deviation from full configuration interaction has been computed, few properties have been carefully investigated as a function of molecular geometry. Here the dipole, quadrupole, and octupole moments are computed for a range of molecular geometries. The addition of Erdahl's T2 condition [Int. J. Quantum Chem. 13, 697 (1978)] to the D, Q, and G conditions produces dipole and multipole moments that agree with full configuration interaction in a double-zeta basis set at all internuclear distances.

Journal Article↗
PubMed2000

Computational method for general multicenter electronic structure calculations.

Here a three-dimensional fully numerical (i.e., chemical basis-set free) method [P. F. Batcho, Phys. Rev. A 57, 6 (1998)], is formulated and applied to the calculation of the electronic structure of general multicenter Hamiltonian systems. The numerical method is presented and applied to the solution of Schrödinger-type operators, where a given number of nuclei point singularities is present in the potential field. The numerical method combines the rapid "exponential" convergence rates of modern spectral methods with the multiresolution flexibility of finite element methods, and can be viewed as an extension of the spectral element method. The approximation of cusps in the wave function and the formulation of multicenter nuclei singularities are efficiently dealt with by the combination of a coordinate transformation and a piecewise variational spectral approximation. The complete system can be efficiently inverted by established iterative methods for elliptical partial differential equations; an application of the method is presented for atomic, diatomic, and triatomic systems, and comparisons are made to the literature when possible. In particular, local density approximations are studied within the context of Kohn-Sham density functional theory, and are presented for selected subsets of atomic and diatomic molecules as well as the ozone molecule.

Computer Simulation↗
PubMed2002

Exact asymptotic results for persistence in the Sinai model with arbitrary drift.

We obtain exact asymptotic results for the disorder averaged persistence of a Brownian particle moving in a biased Sinai landscape. We employ a method that maps the problem of computing the persistence to the problem of finding the energy spectrum of a single-particle quantum Hamiltonian, which can be subsequently found. Our method allows us analytical access to arbitrary values of the drift (bias), thus going beyond the previous methods that provide results only in the limit of vanishing drift. We show that on varying the drift the persistence displays a variety of rich asymptotic behaviors including, in particular, interesting qualitative changes at some special values of the drift.

Journal Article↗
PubMed2005

Multisplitter interaction for entanglement distribution.

In protocols of distributed quantum information processing, a network of bilateral entanglement is a key resource for efficient communication and computation. We propose a model, efficient both in finite and infinite Hilbert spaces, that performs entanglement distribution among the elements of a network without local control. In the establishment of entangled channels, our setup requires only the proper preparation of a single elected element. We suggest a setup of electromechanical systems to implement our proposal.

Journal Article↗
PubMed2006

Graviton propagator from background-independent quantum gravity.

We study the graviton propagator in Euclidean loop quantum gravity. We use spin foam, boundary-amplitude, and group-field-theory techniques. We compute a component of the propagator to first order, under some approximations, obtaining the correct large-distance behavior. This indicates a way for deriving conventional spacetime quantities from a background-independent theory.

Journal Article↗
PubMed2000

Analysis of synaptic quantal depolarizations in smooth muscle using the wavelet transform.

The time-frequency characteristics of synaptic potentials contain valuable information about the process of neurotransmission between nerves and their target organs. For example, at the synapse between autonomic nerves and smooth muscle, two central issues of neurophysiology, i.e., 1) the probability of neurotransmitter release and 2) the quantal behavior of transmission can be deduced from analysis of the rising phases of evoked excitatory junction potentials (eEJP's) recorded from smooth muscle. eEJP rising phases are marked by prominent inflexions, which reflect these features of neuronal activity. Since these inflexions contain time-varying frequency information, we have applied recent techniques of time-frequency analysis based upon wavelet transforms to eEJP's recorded from the guinea-pig vas deferens in vitro. We find that these techniques allow accurate and convenient characterization of neuronal release sites, and that their probability of release falls between 0.001-0.004. We have also analyzed eEJP's recorded in the presence of the chemical 1-heptanol, which reveals quantal depolarizations. These results have helped clarify the nature of the quantal depolarizations that underly eEJP's. The present method offers significant advantages over those previously employed for these tasks, and holds promise as a novel approach to the analysis of synaptic potentials.

Animals↗
PubMed2004

[Reduction of radiation exposure in CT perfusion study using a quantum de-noising filter].

CT perfusion study requires repeatedly scanning the same part of the patient's head, resulting in an increase of local radiation exposure. The purpose of this study was to assess the feasibility of the ultra-low-dose technique with a quantum de-noising filter. The newly developed quantum de-noising filter selectively reduced noise by two-thirds, while maintaining spatial resolution. The low-dose protocol using the quantum de-noising filter and slow rotation speed accomplished a 68% reduction in the local radiation dose compared with the previous standard protocol. The quantum de-noising filter is considered to be useful to reduce radiation exposure and to improve image quality in CT perfusion study.

Brain↗