Oscillator Basis Functions Research Articles

Variational vibrational states of HCOOH

Vibrational states of the formic acid molecule are converged using the GENIUSH–Smolyak approach and the potential energy surface taken from Tew and Mizukami (2016). The quantum nuclear motion is described by using the cis-trans torsional coordinate and eight curvilinear normal coordinates defined with respect to an instantaneous reference configuration changing as a function of the torsional degree of freedom. Harmonic oscillator basis functions are used for the curvilinear normal coordinates, a Fourier basis for the torsional coordinate, and a simple basis pruning condition is combined with a Smolyak integration grid. Trans, cis, and delocalized vibrational states are reported up to and slightly beyond the isomerization barrier.

Structure of the ground and excited states in [formula omitted]Be nucleus

We investigate properties of bound and resonance states in the Λ9Be nucleus. To reveal the nature of these states, we use a three-cluster 2α+Λ microscopic model. The model incorporates Gaussian and oscillator basis functions and reduces a three-cluster Schrödinger equation to a two-body like many-channel problem with the two-cluster subsystems (Λ5He and 8Be) being in a bound or a pseudo-bound state. Influence of the cluster polarization on the energy and widths of resonance states in Λ9Be and on elastic and inelastic Λ5He +α scattering is analyzed.

Chiral three-body force and monopole properties of shell-model Hamiltonian

So far, the nature of three-nucleon forces (3NFs) derived by the chiral effective field theory has been intensively investigated by various theoretical approaches. In this work, to address the chiral 3NF within the shell-model framework, three-body matrix elements are formulated in terms of the harmonic oscillator basis functions, by adopting the nonlocal regulator. We perform a benchmark test for p-shell nuclei inorder to confirm our framework. Then we show that the contribution of the 3NF to the monopole component of the effective shell model Hamiltonian plays an essential role to account for the shell evolution of f p-shell nuclei.

Microscopic three-cluster model of 10Be

We investigate spectrum of bound and resonance states in 10Be, and scattering of alpha-particles on 6He. For this aim we make use of a three-cluster microscopic model. This model incorporates Gaussian and oscillator basis functions and reduces three-cluster Schrödinger equation to a two-body like many-channel problem with the two-cluster subsystem being in a bound or a pseudo-bound state. Much attention is given to the effects of cluster polarization on spectrum of bound and resonance states in 10Be, and on elastic and inelastic He6+α scattering.

Variational analysis of mass spectra and decay constants for ground state pseudoscalar and vector mesons in the light-front quark model

Using the variational principle, we compute mass spectra and decay constants of ground state pseudoscalar and vector mesons in the light-front quark model (LFQM) with the QCD-motivated effective Hamiltonian including the hyperfine interaction. By smearing out the Dirac delta function in the hyperfine interaction, we avoid the issue of negative infinity in applying the variational principle to the computation of meson mass spectra and provide analytic expressions for the meson mass spectra. Our analysis with the smeared hyperfine interaction indicates that the interaction for the heavy meson sector including the bottom and charm quarks gets more point-like. We also consider the flavor mixing effect in our analysis and determine the mixing angles from the mass spectra of $(\omega,\phi)$ and $(\eta,\eta')$. Our variational analysis with the trial wave function including the two lowest order harmonic oscillator basis functions appears to improve the agreement with the data of meson decay constants and the heavy meson mass spectra over the previous computation handling the hyperfine interaction as perturbation.

Resonant tunneling of a few-body cluster through repulsive barriers

A model for quantum tunnelling of a cluster comprised of A identical particles, interacting via oscillator-type potential, through short-range repulsive barrier potentials is introduced for the first time in symmetrized-coordinate representation and numerically studied in the s-wave approximation. A constructive method for symmetrizing or antisymmetrizing the (A − 1)-dimensional harmonic oscillator basis functions in the new symmetrized coordinates with respect to permutations of coordinates of A identical particles is described. The effect of quantum transparency, manifesting itself in nonmonotonic resonance-type dependence of the transmission coefficient upon the energy of the particles, their number A = 2, 3, 4 and the type of their symmetry, is analyzed. It is shown that the total transmission coefficient demonstrates the resonance behavior due to the existence of barrier quasi-stationary states, embedded in the continuum.

Shape and Size Dependent Electronic Properties of GaAs/AlGaAs Quantum Dots

We have theoretically investigated the effect of shape anisotropy and size on electronic structure of GaAs/AlGaAs quantum dots (QDs). The quantum dot is modeled by anisotropic parabolic confinement potential in the plane perpendicular to the growth direction while the confinement along the growth direction is modeled as quantum well potential. The Luttinger Hamiltonian formulation has been used to account for the valence subband mixing. The electronic structure is calculated by numerical diagonalization of Luttinger Hamiltonian using the harmonic oscillator basis functions. The calculations for hole energies and transition energies have been carried out over wide range of size and shape of QDs. The results show that transition energy of QDs decreases with the height of QDs. Significant variation in the hole energy is observed with the change in anisotropy. We also observe that shape anisotropy and mixing have significant effect on the energy states.

Anharmonic oscillator and the optimized basis expansion

We introduce various optimization schemes for highly accurate calculation of the eigenvalues and the eigenfunctions of the one-dimensional anharmonic oscillators. We present several methods of analytically fixing the nonlinear variational parameter specified by the domain of the trigonometric basis functions. We show that the optimized parameter enables us to determine the energy spectrum to an arbitrary accuracy. Also, using the harmonic oscillator basis functions, we indicate that the resulting optimal frequency agrees with the one obtained by the principle of the minimal sensitivity.

Recurrence relations for one-dimensional harmonic oscillator matrix elements of Gaussian and exponential operators

This paper reports the development of several general recurrence relations that can be used to evaluate one-dimensional, three centre harmonic oscillator matrix elements of the operators and f = exp(−cx C ). The matrix elements have the general form ⟨φ m (a 1/2 x A )|g(or f)|φ n (b 1/2 x B )⟩; φ m is the harmonic oscillator basis function for an eigenstate m. The coordinates are x A = x − A x , and so on, where A x , B x , and C x are points of reference for the displacement of a common atom whose instantaneous coordinate is x. A typical case might be that of a hydrogen atom referred to two wells located at A x and B x , and a second atom located at C x on the x axis. The recurrence relations apply to all cases including the two centre A x = B x and overlap integrals, A x ≠ B x , c = 0, and C x = 0. Moreover, the recurrence relations can generate matrix elements to any order. The applications of some of these recursions are illustrated with several examples: (1) the variational treatment of the Morse oscillator using one-dimensional harmonic oscillator basis functions; (2) the development of a model of the Morse oscillator in Gaussian coordinates together with (3) the variational analysis of that model. In addition, (4) the simplest version of a symmetric double potential well system is examined using both the Morse oscillator and the model potential.

Microscopic cluster description of12C

We investigate both bound and resonance states in 12C embedded in a three-\alpha-cluster continuum using two distinct three-cluster microscopic models. The first one relies on the Hyperspherical Harmonics basis to enumerate the channels describing the three-cluster continuum. The second model incorporates both Gaussian and Oscillator basis functions, and reduces the three-cluster problem to a two-cluster one, in which a two-cluster subsystem is described by a set of pseudo-bound state states. It is shown that the results agree well with comparable calculations from the literature.

Accurate multimode vibrational calculations using a B-spline basis: theory, tests and application to dioxirane and diazirinone

The use of B-spline basis sets is explored in the context of a vibrational program for automatic potential energy surface (PES) construction and multimode anharmonic vibrational wave function calculation. Results are compared with calculations using localized Gaussians and harmonic oscillator basis functions. Potential energy surfaces are constructed in an iterative fashion using a recently developed adaptive density-guided approach. The basis set requirements for an accurate representation of the vibrational wave functions are met by both B-spline basis sets as well as the well-known distributed Gaussian basis sets. Furthermore, the property of minimal support of the B-spline functions makes the use of B-spline basis more advantageous compared to harmonic oscillator basis functions, when combined with the adaptive procedure for PES construction used in this work. The methodology is tested for model potentials and water and subsequently applied to study vibrational states of dioxirane and diazirinone. The latter have proven to be elusive to experimental characterization and high level vibrational calculations based on accurate PES may offer a guidance for the experimental work.

Assigning quantum labels to variationally computed rotational-vibrational eigenstates of polyatomic molecules

A procedure is investigated for assigning physically transparent, approximate vibrational and rotational quantum labels to variationally computed eigenstates. Pure vibrational wave functions are analyzed by means of normal-mode decomposition (NMD) tables constructed from overlap integrals with respect to separable harmonic oscillator basis functions. Complementary rotational labels J(K(a)K(c)) are determined from rigid-rotor decomposition (RRD) tables formed by projecting rotational-vibrational wave functions (J not equal 0) onto products of symmetrized rigid-rotor basis functions and previously computed (J=0) vibrational eigenstates. Variational results for H(2)O, HNCO, trans-HCOD, NCCO, and H(2)CCO are presented to demonstrate the NMD and RRD schemes. The NMD analysis highlights several resonances at low energies that cause strong mixing and cloud the assignment of fundamental vibrations, even in such simple molecules. As the vibrational energy increases, the NMD scheme documents and quantifies the breakdown of the normal-mode model. The RRD procedure proves effective in providing unambiguous rotational assignments for the chosen test molecules up to moderate J values.

A relativistic extension of the complex scaling method using oscillator basis functions

We develop the complex scaling method within the relativistic framework by expanding the Dirac spinors in the complete set of eigensolutions of a harmonic oscillator potential, and present the theoretical formalism of describing the discrete bound and resonant states on the same footing. Based on a well established and frequently used model, we demonstrate the utility and applicability of the extended method and examine the stability of the results with respect to the variations of the parameters of the model. Satisfactory agreements are found for all the calculated results in comparison with some other calculations in references. Especially, the present calculation in the nonrelativistic limit gives a consistent result with that in the nonrelativistic calculation.

Effective interactions, large-scale diagonalization, and one-dimensional quantum dots

The widely used large-scale diagonalization method using harmonic oscillator basis functions (an instance of the Rayleigh-Ritz method [S. Gould, Variational Methods for Eigenvalue Problems: An Introduction to the Methods of Rayleigh, Ritz, Weinstein, and Aronszajn (Dover, New York, 1995)], also called a spectral method, configuration-interaction method, or ``exact diagonalization'' method) is systematically analyzed using results for the convergence of Hermite function series. We apply this theory to a Hamiltonian for a one-dimensional model of a quantum dot. The method is shown to converge slowly, and the nonsmooth character of the interaction potential is identified as the main problem with the chosen basis, while, on the other hand, its important advantages are pointed out. An effective interaction obtained by a similarity transformation is proposed for improving the convergence of the diagonalization scheme, and numerical experiments are performed to demonstrate the improvement. Generalizations to more particles and dimensions are discussed.

Franck-Condon factors based on anharmonic vibrational wave functions of polyatomic molecules

Franck-Condon (FC) integrals of polyatomic molecules are computed on the basis of vibrational self-consistent-field (VSCF) or configuration-interaction (VCI) calculations capable of including vibrational anharmonicity to any desired extent (within certain molecular size limits). The anharmonic vibrational wave functions of the initial and final states are expanded unambiguously by harmonic oscillator basis functions of normal coordinates of the respective electronic states. The anharmonic FC integrals are then obtained as linear combinations of harmonic counterparts, which can, in turn, be evaluated by established techniques taking account of the Duschinsky rotations, geometry displacements, and frequency changes. Alternatively, anharmonic wave functions of both states are expanded by basis functions of just one electronic state, permitting the FC integral to be evaluated directly by the Gauss-Hermite quadrature used in the VSCF and VCI steps [Bowman et al., Mol. Phys. 104, 33 (2006)]. These methods in conjunction with the VCI and coupled-cluster with singles, doubles, and perturbative triples [CCSD(T)] method have predicted the peak positions and intensities of the vibrational manifold in the X 2B1 photoelectron band of H2O with quantitative accuracy. It has revealed that two weakly visible peaks are the result of intensity borrowing from nearby states through anharmonic couplings, an effect explained qualitatively by VSCF and quantitatively by VCI, but not by the harmonic approximation. The X 2B2 photoelectron band of H2CO is less accurately reproduced by this method, likely because of the inability of CCSD(T)/cc-pVTZ to describe the potential energy surface of open-shell H2CO+ with the same high accuracy as in H2O+.

Piecewise moments method: Generalized Lanczos technique for nuclear response surfaces

For some years Lanczos moments methods have been combined with large-scale shell-model calculations in evaluations of the spectral distributions of certain operators. This technique is of great value because the alternative, a state-by-state summation over final states, is generally not feasible. The most celebrated application is to the Gamow-Teller operator, which governs beta decay and neutrino reactions in the allowed limit. The Lanczos procedure determines the nuclear response along a line q=0 in the (omega,q) plane, where omega and q are the energy and three-momentum transfered to the nucleus. However, generalizing such treatments from the allowed limit to general electroweak response functions at arbitrary momentum transfers seems considerably more difficult: the response function must be determined over the entire (omega,q) plane for an operator O(q) that is not fixed, but depends explicitly on q. Such operators arise in any semileptonic process where the momentum transfer is comparable to (or larger than) the inverse nuclear size. Here we show, for Slater determinants built on harmonic oscillator basis functions, that the nuclear response for any multipole operator O(q) can be determined efficiently over the full response plane by a generalization of the standard Lanczos moments method. We describe the Piecewise Moments Method and thoroughly explore its convergence properties for the test case of electromagnetic responses in a full sd-shell calculation of 28Si. We discuss possible extensions to a variety of electroweak processes, including charged- and neutral-current neutrino scattering.

Analysis of Local Anharmonicity Using Gaussian Model Potentials and Cartesian Oscillator Basis Sets: Example, HCN

This paper examines local anharmonic vibrations in molecules using an analysis that starts with an ab initio potential energy surface, fits a model potential constructed of Gaussian basis functions, and proceeds to a quantum mechanical analysis of the anharmonic modes using Cartesian harmonic oscillator basis functions in a variational calculation. The objective of this work is to suggest methods, with origins in nuclear and molecular (electronic) quantum mechanics, that should be useful for the accurate analysis of the local anharmonic motions of hydrogen, and perhaps other atoms or small molecular fragments, residing in molecularly complicated but otherwise harmonic environments.

Harmonic oscillator basis functions and Gaussian model potentials for the analysis of anharmonic vibrations*

AbstractPotential energy surfaces are mapped onto expansions in Gaussian basis functions in order to examine vibrational anharmonicity in molecules; the Gaussian expansions are constructed to take many‐body effects into account. Subsequent quantum mechanical analyses using harmonic oscillator basis sets are carried out. The appropriate matrix elements needed for the analyses follow using Talmi transformations and several standard integrals involving harmonic oscillator basis functions. Local anharmonicity can be explored with relatively modest expansions of the potential energy surfaces. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

TWO-OSCILLATOR BASIS EXPANSION FOR THE SOLUTION OF RELATIVISTIC MEAN FIELD EQUATIONS

In the relativistic mean field (RMF) calculations usually the basis expansion method is employed. For this one uses single harmonic oscillator (HO) basis functions. A proper description of the ground state nuclear properties of spherical nuclei requires a large (around 20) number of major oscillator shells in the expansion. In halo nuclei where the nucleons have extended spatial distributions, the use of single HO basis for the expansion is inadequate for the correct description of the nuclear properties, especially that of the surface region. In order to rectify these inadequacies, in the present work an orthonormal basis composed of two HO basis functions having different sizes is proposed. It has been shown that for a typical case of (A=11) the ground state constructed using two-HO wave functions extends much beyond the second state or even third excited state of the single HO wave function. To demonstrate its usefulness explicit numerical RMF calculations have been carried out using this procedure for a set of representative spherical nuclei ranging from 16 O to 208 Pb . The binding energies, charge radii and density distributions have been correctly reproduced in the present scheme using a much smaller number of major shells (around 10) in the expansion.

Internal coordinate Hamiltonian model for Fermi resonances and local modes in methane

A vibrational model which is based on a Hamiltonian expressed in terms of curvilinear internal coordinates is applied to the overtone spectrum of methane, CH4. Symmetrized internal coordinates and their conjugate momenta are used as the bending variables. The stretching part of the Hamiltonian is expressed in an unsymmetrized form. Both the kinetic operator and the potential energy function are expanded as Taylor series around the equilibrium configuration. Symmetrized local mode basis functions for the stretches and symmetrized two- and three-dimensional harmonic oscillator basis functions in the Cartesian representations for bending degrees of freedom are used. Only resonance couplings are taken into account. Apart from some standard diagonal contributions harmonic oscillator matrix elements have been employed. This results in a simple block diagonal Hamiltonian model. The nonlinear least squares method is used to optimize model parameters for 12CH4. Observed vibrational term values up to 6050 cm−1 are included as data. Potential energy parameters obtained from the Hamiltonian parameters agree well with a previously published anharmonic force field calculation. A unitary transformation between internal coordinate and normal coordinate representations is found to provide simple interpretations for the standard normal mode theory based spectroscopic parameters.