Full Quantum Mechanical Calculations Research Articles

Reactive excitation functions for F+p-H2/n-H2/D2 and the vibrational branching for F+HD

Complementary to our recent report on the F+HD reaction, the reactive excitation functions for the other isotopomers are presented. Through analysis of the differential cross section data, the collisional energy dependencies of product vibrational branchings for F+HD are also reported here. Several important conclusions can be drawn from this work. First, the transition-state properties, in particular the barrier height, of this reaction are well-characterized by the SW PES, despite its neglect of spin–orbit couplings. Second, contrary to the theoretical conclusion in recent literatures, an experimental observation is presented which seems to suggest that a resonance may indeed exist for the F+H2 reaction in support of the original interpretation proposed by Lee and co-workers. Third, the vibrational branching for the F+HD→HF+D reaction elucidates another facet of resonance effects in the integral cross sections. Finally, the nonadiabatic reactivity of the spin–orbit excited F*(2P1/2) atom is found to be small, which is in line with the conclusion inferred from a most recent, full quantum mechanical multisurface calculation.

Quantum dynamics of model proton-coupled electron transfer reactions

We present detailed studies of the quantum dynamics of model proton-coupled electron transfer (PCET) reactions in solution. We compared the full quantum mechanical calculations with the mean field approaches for the solvent. A multiple time-scale wavepacket propagation method is used for simulating the combined electron/proton/solvent dynamics. The dynamics of the model PCET reactions are found to exhibit extensive mixing of the proton/electron adiabatic states. Such nonadiabatic couplings between adiabatic states are not properly described in mean field calculations except for at short times. The time-scale differences arising from the mass disparities of the different degrees of freedom can affect the detailed dynamics of the model PCET reactions, as illustrated by subtle differences in the sequential PT→ET and ET→PT mechanisms.

Nonadiabatic photodissociation dynamics of ICN in the à continuum: A semiclassical initial value representation study

In this paper we investigate the nonadiabatic photodissociation dynamics of ICN in the à continuum, using a semiclassical initial value representation method which is able to describe electronically nonadiabatic processes through the quantization of the classical electron–nuclear model Hamiltonian of Meyer and Miller [J. Chem. Phys. 70, 3214 (1979)]. We explore the capabilities of this semiclassical technique as applied to studying the ICN absorption spectrum, and the CN rotational distribution, through direct comparison of our semiclassical results with experimental data, and with full quantum mechanical calculations. We find that the Meyer–Miller Hamiltonian, quantized according to the semiclassical prescription, describes the ICN photodissociation dynamics in excellent agreement with full-quantum mechanical calculations.

Barrier height engineering on GaAs THz Schottky diodes by means of high-low doping, InGaAs- and InGaP-layers

Barrier height engineering of n-GaAs-based millimeter-wave Schottky diodes using strained InGaAs/GaAs and InGaP/GaAs heterostructures and a high doping surface layer is presented. The Schottky barrier height can be varied between /spl Phi//sub fb/=0.52 eV and /spl Phi//sub fb/=1.0 eV. The use of a pseudomorphic InGaAs layer and/or a thin high doping layer at the surface significantly reduces the Schottky barrier height. This is advantageous for low-drive zero bias mixing applications, A full quantum mechanical numerical calculation is presented to simulate the influence of different high doping layer thicknesses on the diode's dc characteristic. The theoretical results are compared with experimental results, For reverse bias applications (e.g., varactors) a barrier height and breakdown voltage enhancement is realized with a lattice matched InGaP/GaAs heterostructure. The barrier height value is determined by temperature dependent dc-measurements. The epitaxial layered structures are grown by molecular beam epitaxy. The diode devices are fabricated in a fully planar technology using selective oxygen implantation for lateral isolation. The diode's cut-off frequencies are in the THz-range.

A semiclassical study of collision-induced dissociation in He+H2: The effect of molecular rotation

Dissociation processes in three-dimensional He+H2 collisions are studied by using a semiclassical approach which treats the relative radial motion in classical mechanics and the other motions (vibration/dissociation and rotation) in quantum mechanics. Centrifugal sudden approximation is assumed to solve the semiclassical equation. The results are compared with previous full quantum mechanical calculations in which infinite order sudden approximation has been applied. The present study shows that the molecular rotation must be accurately taken into account in the dissociation process even at very high collision energies where the energy sudden assumption is usually expected to be satisfied for low molecular rotational states.

Quantum Phonon Fluctuations in Mesoscopic Dimerized Systems

We describe quantitatively the structural change in finite Peierls chains. Full quantum mechanical calculations show: (i) how the smooth phase transition occurs by the continuous change of the ground state wave function which preserves the symmetry of the Hamiltonian; (ii) the structural change is accompanied by an anharmonicity which becomes very pronounced in the broad transition region; (iii) the dimerization is suppressed for sizes shorter than a certain critical size. Quantum phonon fluctuations can make the latter sufficiently large (∼ 0.1 - 1 µm) to be detected in mesoscopic systems ( e.g. perylene radical cation salts) with weaker electron-phonon coupling. As possible techniques for experimental observation, we suggest the nonlinear optical absorption and the ultrasonic attenuation. Our calculations are based on numerical diagonalization done exactly in small chains and within the adiabatic ansatz for longer chains. We indicate how to test selfconsistently the latter results and show that they are...

A unitary convolution approximation for the impact-parameter dependent electronic energy loss

In this work, we propose a simple method to calculate the impact-parameter dependence of the electronic energy loss of bare ions for all impact parameters. This perturbative convolution approximation (PCA) is based on first-order perturbation theory, and thus, it is only valid for fast particles with low projectile charges. Using Bloch's stopping-power result and a simple scaling, we get rid of the restriction to low charge states and derive the unitary convolution approximation (UCA). Results of the UCA are then compared with full quantum-mechanical coupled-channel calculations for the impact-parameter dependent electronic energy loss.

Semiclassical molecular dynamics simulations of excited state double-proton transfer in 7-azaindole dimers

An ab initio excited state potential energy surface is constructed for describing excited state double proton transfer in the tautomerization reaction of photo-excited 7-azaindole dimers, and the ultrafast dynamics is simulated using the semiclassical (SC) initial value representation (IVR). The potential energy surface, determined in a reduced dimensionality, is obtained at the CIS level of quantum chemistry, and an approximate version of the SC-IVR approach is introduced which scales linearly with the number of degrees of freedom of the molecular system. The accuracy of this approximate SC-IVR approach is verified by comparing our semiclassical results with full quantum mechanical calculations. We find that proton transfer usually occurs during the first intermonomer symmetric-stretch vibration, about 100 fs after photoexcitation of the system, and produces an initial 15 percent population decay of the reactant base-pair, which is significantly reduced by isotopic substitution.

A pseudobond approach to combining quantum mechanical and molecular mechanical methods

A major challenge for combined quantum mechanical and molecular mechanical methods (QM/MM) to study large molecules is how to treat the QM/MM boundary that bisects some covalent bonds. Here a pseudobond approach has been developed to solve this problem for ab initio QM/MM calculations: a one-free-valence atom with an effective core potential is constructed to replace the boundary atom of the environment part and to form a pseudobond with the boundary atom of the active part. This pseudobond, which is described only by the QM method, is designed to mimic the original bond with similar bond length and strength, and similar effects on the rest of the active part. With this pseudobond approach, some well-known deficiencies of the link atom approach have been circumvented and a well-defined potential energy surface of the whole QM/MM system has been provided. The construction of the effective core potential for the pseudobond is independent of the molecular mechanical force field and the same effective core potential is applicable to both Hartree–Fock and density functional methods. Tests on a series of molecules yield very good structural, electronic, and energetic results in comparison with the corresponding full ab initio quantum mechanical calculations.

Direct evaluation of the Pauli repulsion energy using `classical' wavefunctions in hybrid quantum/classical potential energy surfaces

A new procedure for evaluating the Pauli repulsion energy in hybrid quantum/classical treatments is discussed and applied to two model systems: the hydrogen-bonded water dimer and the charged system H 2O/Li +. The Pauli repulsion is evaluated by associating temporary wavefunctions with the classical region and then exploiting the equivalence of Pauli exclusion and permutational antisymmetry. The proposed hybrid quantum/classical treatment is devoid of any empirical parameters. Comparison with full quantum mechanical calculations suggests that the method may be applied to a wide variety of systems.

Magnetic breakdown and Landau level spectra of a tunable double-quantum-well Fermi surface

By measuring longitudinal resistance, we map the Landau level spectra of double quantum wells as a function of both parallel ( B ‖) and perpendicular ( B ⊥) magnetic fields. In this continuously tunable highly non-parabolic system, the cyclotron masses of the two Fermi surface orbits change in opposite directions with B ‖. This causes the two corresponding ladders of Landau levels formed at finite B ⊥ to exhibit multiple crossings. We also observe a third set of Landau levels, independent of B ‖, which arise from magnetic breakdown of the Fermi surface. Both semiclassical and full-quantum mechanical calculations show good agreement with the data.

Double-layered quantum dots in a magnetic field: The ground state and the far-infrared response

We investigate the ground-state properties, the far-infrared (FIR) response, and the collective modes of a square lattice of double-layered quantum dots by applying classical and quantum-mechanical concepts. Using classical self-consistent linear response theory for the dot array we derive analytic results for the magnetoabsorption spectrum and the frequencies, linewidths, and oscillator strengths of the optical and acoustic collective modes including the effect of intercell and intracell interactions. Within the full quantum-mechanical calculation (applying density-functional theory with the local-density approximation) for a single double-layered quantum dot we obtain numerically ground-state properties and the FIR excitation frequencies. We use a quantum dot model with a realistic distribution of background charge, which accounts for surface states and total charge neutrality. In both approaches we study the dependence of the oscillator strengths of the acoustic mode on the asymmetry of the double-layered system in order to give information on the condition for experimental detection.

Water on the sun: molecules everywhere.

[ Takeshi Oka ][1] In 1995, water was found to exist on the sun when infrared absorption bands in sun spots were analyzed [ Science 268 , 1155 (1995)]. In his Perspective, Oka discusses new results in the same issue by [ Polyansky et al .][2] on computational analysis of the spectrum of hot water. The computer calculations confirm the assignment of the infrared spectrum as arising from water, and reveal some interesting new features about the water spectrum. As more powerful computers become available, it may be possible to eventually perform full quantum mechanical calculations of large polyatomic molecules. [][3] [1]: http://www.sciencemag.org#affiliation [2]: http://www.sciencemag.org/cgi/content/short/277/5324/346 [3]: http://

Rydberg wave packets in parallel electric and magnetic fields

The magnitude of the time autocorrelation function M between states excited by two Gaussian laser pulses is calculated for both hydrogen and rubidium atoms in parallel electric and magnetic fields. M is determined by a full quantum-mechanical calculation but the peaks are identified with the periods of the shortest periodic orbits of the corresponding classical system. Qualitative agreement is obtained with experimental results, however, discrepancies are found in the relative heights of the peaks.

Interaction of He and Ne with Cu surfaces

The extensive data available for He and Ne beam elastic scattering from Cu surfaces are analysed by full quantum mechanical calculations and by describing the atom-surface interaction potential according to the present knowledge on van der Waals pair-potentials. It is shown that Ne penetrates the surface electron cloud much deeper than He and that the ratio of Ne-to-He adsorption energies exceeds the ratio of the atomic polarizabilities.

Quantitative simulation of a resonant tunneling diode

Quantitative simulation of an InGaAs/InAlAs resonant tunneling diode is obtained by relaxing three of the most widely employed assumptions in the simulation of quantum devices. These are the single band effective mass model (parabolic bands), Thomas-Fermi charge screening, and the Esaki-Tsu 1D integral approximation for current density. The breakdown of each of these assumptions is examined by comparing to the full quantum mechanical calculations of self-consistent quantum charge in a multiband basis explicitly including the transverse momentum.

Beyond the eikonal model for few-body systems

A physical prescription to improve the accuracy of few-body Glauber model calculations of reactions involving loosely bound projectiles is presented in which the eikonal phase shift function of each projectile constituent is modified to account for curvature of its trajectory. Noneikonal effects due to both nuclear and Coulomb interactions are treated on an equal footing. The proposed method is assessed quantitatively by comparison with full quantum mechanical calculations in the case of ${}^{11}$Be+${}^{12}$C elastic scattering, treated as a three-body ${}^{10}$Be+$n$+target problem, at energies of 25 and 49.3 MeV/nucleon. Calculated cross-section angular distributions which include the noneikonal modifications are shown to be accurate to larger scattering angles, and for lower incident projectile energies.

Electronic states in antidot lattices: Scattering-matrix formalism.

A method of full quantum-mechanical calculation of the energy bands with the use of the S matrix is developed in antidot lattices subjected to a uniform perpendicular magnetic field. It can simplify the calculations considerably in comparison with other methods because only several traveling and a few evanescent modes are sufficient to give accurate results. The resulting energy bands are extremely complicated for realistic antidots. Calculated density of states are analyzed semiclassically in terms of the periodic orbit theory. \textcopyright{} 1996 The American Physical Society.

Astrophysical S-factor of the 7Be( p, γ) 8B reaction from Coulomb dissociation of 8B

The Coulomb dissociation method to obtain the astrophysical S-factor, S 17(0), for the 7Be( p, γ) 8B reaction at solar energies is investigated by analysing the recently measured data on the breakup reaction 208Pb( 8B, 7Be p) 208Pb at 46.5 MeV/A beam energy. Breakup cross sections corresponding to E1, E2 and M1 transitions are calculated with a theory of Coulomb excitation that includes the effects of the Coulomb recoil as well as relativistic retardation. The interplay of nuclear and Coulomb contributions to the breakup process is studied by performing a full quantum mechanical calculation within the framework of the distorted-wave Born approximation. In the kinematical regime of the present experiment, both nuclear as well as Coulomb-nuclear interference processes affect the pure Coulomb breakup cross sections very marginally. The E2 cross sections are strongly dependent on the model used to describe the structure of 8B. The value of S 17(0) is deduced with and without E2 and M1 contributions added to the E1 cross sections and the results are discussed.

Ab initio molecular orbital calculations of solvent clusters of trans-N-methylacetamide: structure, ring cluster formation and out-of-plane deformation

The solvation of trans amides has been investigated by the use of full gradient optimization ab initio quantum mechanical calculation techniques. The complexes have been determined at the Hartree–Fock (HF) level with a 4-31G*/4-31G** basis set and at the second-order Moller–Plesset perturbation (MP2) level. Three NMA–water clusters were investigated: trans-NMA with two molecules of water forming a ring cluster at the amide oxygen; trans-NMA with two molecules of water at the amide oxygen forming hydrogen bonds along the direction of the lone-pair electrons; trans-NMA with one molecule of water at the CO group and one at the NH group. In addition, 4-31G* basis set calculations for trans-NMA with two molecules of acetonitrile were performed. The CO⋯H(W) hydrogen bond lengths, electron-density population analysis and molecular-orbital analysis of trans-NMA with two molecules of water at the amide oxygen demonstrate the importance of concurrent water–water and water–(carbonyl) oxygen hydrogen-bond interactions. The complex of trans-NMA with two molecules of water forming a ring cluster at the amide oxygen indicates the formation of a non-planar amide bond and the generation of a chiral centre at the amide nitrogen; this structure has a 5 % Boltzmann distribution at room temperature at the MP2 level. Vibrational-frequency analysis shows that its hydrogen-bonded water molecules are vibrationally coupled. Orbital analysis suggests that there is a considerable solute-occupied space reorganization caused by the rearrangement of the water solvent molecules. Comparisons are made with previous theoretical studies of amide–water interactions and experimental spectroscopic, X-ray and neutron-diffraction data on the hydration of amides, peptides and proteins.