2,999 research outputs found
The higher-order C_n dispersion coefficients for hydrogen
The complete set of 2nd, 3rd and 4th order van der Waals coefficients, C_n up
to n=32 for the H(1s)-H(1s) dimer are computed using pseudo-states to evaluate
the appropriate sum rules. A study of the convergence pattern for n<=16
indicates that all the C_n (n<=16) coefficients are accurate to 13 significant
digits. The relative size of the 4th-order C^4_n to the 2nd-order C^2_n
coefficients is seen to increase as n increases and at n=32 the 4th-order term
is actually larger.Comment: 5 pages under review PR
Convergence of CI single center calculations of positron-atom interactions
The Configuration Interaction (CI) method using orbitals centered on the
nucleus has recently been applied to calculate the interactions of positrons
interacting with atoms. Computational investigations of the convergence
properties of binding energy, phase shift and annihilation rate with respect to
the maximum angular momentum of the orbital basis for the e^+Cu and PsH bound
states, and the e^+-H scattering system were completed. The annihilation rates
converge very slowly with angular momentum, and moreover the convergence with
radial basis dimension appears to be slower for high angular momentum. A number
of methods of completing the partial wave sum are compared, an approach based
on a Delta X_J = a/(J + 1/2)^n + b/(J + 1/2)^(n+1) form (with n = 4 for phase
shift (or energy) and n = 2 for the annihilation rate) seems to be preferred on
considerations of utility and underlying physical justification.Comment: 23 pages preprint RevTeX, 11 figures, submitted to PR
Positronic complexes with unnatural parity
The structure of the unnatural parity states of PsH, LiPs, NaPs and KPs are
investigated with the configuration interaction and stochastic variational
methods. The binding energies (in hartree) are found to be 8.17x10-4,
4.42x10-4, 15.14x10-4 and 21.80x10-4 respectively. These states are constructed
by first coupling the two electrons into a configuration which is predominantly
3Pe, and then adding a p-wave positron. All the active particles are in states
in which the relative angular momentum between any pair of particles is at
least L = 1. The LiPs state is Borromean since there are no 3-body bound
subsystems (of the correct symmetry) of the (Li+, e-, e-, e+) particles that
make up the system. The dominant decay mode of these states will be radiative
decay into a configuration that autoionizes or undergoes positron annihilation.Comment: 10 pages RevTeX, 6 figures, in press Phys.Rev.
Effective oscillator strength distributions of spherically symmetric atoms for calculating polarizabilities and long-range atom-atom interactions
Effective oscillator strength distributions are systematically generated and
tabulated for the alkali atoms, the alkaline-earth atoms, the alkaline-earth
ions, the rare gases and some miscellaneous atoms. These effective
distributions are used to compute the dipole, quadrupole and octupole static
polarizabilities, and are then applied to the calculation of the dynamic
polarizabilities at imaginary frequencies. These polarizabilities can be used
to determine the long-range , and atom-atom interactions
for the dimers formed from any of these atoms and ions, and we present tables
covering all of these combinations
Large dimension Configuration Interaction calculations of positron binding to the group II atoms
The Configuration Interaction (CI) method is applied to the calculation of
the structures of a number of positron binding systems, including e+Be, e+Mg,
e+Ca and e+Sr. These calculations were carried out in orbital spaces containing
about 200 electron and 200 positron orbitals up to l = 12. Despite the very
large dimensions, the binding energy and annihilation rate converge slowly with
l, and the final values do contain an appreciable correction obtained by
extrapolating the calculation to the l to infinity limit. The binding energies
were 0.00317 hartree for e+Be, 0.0170 hartree for e+Mg, 0.0189 hartree for
e+Ca, and 0.0131 hartree for e+Sr.Comment: 13 pages, no figs, revtex format, Submitted to PhysRev
The higher order C_n dispersion coefficients for the alkali atoms
The van der Waals coefficients, from C_11 through to C_16 resulting from 2nd,
3rd and 4th order perturbation theory are estimated for the alkali (Li, Na, K
and Rb) atoms. The dispersion coefficients are also computed for all possible
combinations of the alkali atoms and hydrogen. The parameters are determined
from sum-rules after diagonalizing the fixed core Hamiltonian in a large basis.
Comparisons of the radial dependence of the C_n/r^n potentials give guidance as
to the radial regions in which the various higher-order terms can be neglected.
It is seen that including terms up to C_10/r^10 results in a dispersion
interaction that is accurate to better than 1 percent whenever the
inter-nuclear spacing is larger than 20 a_0. This level of accuracy is mainly
achieved due to the fortuitous cancellation between the repulsive (C_11, C_13,
C_15) and attractive (C_12, C_14, C_16) dispersion forces.Comment: 8 pages, 7 figure
Dispersion coefficients for H and He interactions with alkali-metal and alkaline-earth-metal atoms
The van der Waals coefficients C-6, C-8, and C-10 for H and He interactions with the alkali-metal (Li, Na, K, and Rb) and alkaline-earth-metal (Be, Mg, Ca, and Sr) atoms are determined from oscillator strength sum rules. The oscillator strengths were computed using a combination of ab initio and semiempirical methods. The dispersion parameters generally agree with close to exact variational calculations for Li-H and Li-He at the 0.1% level of accuracy. For larger systems, there is agreement with relativistic many-body perturbation theory estimates of C-6 at the 1% level. These validations for selected systems attest to the reliability of the present dispersion parameters. About half the present parameters lie within the recommended bounds of the Standard and Certain compilation [J. Chem. Phys. 83, 3002 (1985)]
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