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The Multiconfiguration Hartree-Fock method, extended to include relativistic effects in the Breit-Pauli approximation (MCHF + BP), has been used to determine wavefunctions for the different LSJ states of the 2 s 2 2 p z ground and the 2 s 2 p 3 excited configurations of the carbon isoelectronic sequence for nuclear charges, Z , up to thirty. Term energies and fine-structure splitting have been predicted from these wavefunctions, as well as transition probabilities. The latter include the forbidden transitions, electric quadrupole (E2) and magnetic dipole (MI), between the levels of the ground configuration, as well as the allowed 2 s 2 2 p 2 -2 s 2 p 3 transitions. In this paper our results are reported and typical transitions compared with those from other theories as well as those from observation, as available. For Z up to about 20, our energy levels are generally in better agreement with observation than those from other theories. ~ a Garatang [ 181 ' Kastner et al. 191 Physica Scripta 32 BdlUJa and Doyle (101

The multiconfigurationHartree-Fock method of Saha, Pindzola, and Compton [Phys. Rev. A 38, 128 (1988)] applied to photoionization of atoms has been extended in order to consider elastic scattering of electrons from neon atoms. The dynamical polarization and the electron-correlation eff'ects, which are very important in this case, have been taken into account more accurately in the ab initio method through the configuration-interaction procedure. Phase shifts, and differential, integral, and momentum-transfer cross sections for electrons elastically scattered from neon atoms are reported for the impact-energy range from 0.136 to 70 eV. The calculated results are compared with experimental and other theoretical results. It is found that the present multiconfigurational self-consistent-field method yields high-quality results which show excellent agreement with experiment and compare well with other theoretical results.

The scattering length for electron-neon scattering is calculated very accurately for the first time by an ab initio method resulting in a rigorous upper bound of 0.2218ao. The calculation uses the multiconfiguration Hartree-Fock method which includes the effects of dynamical target polarization and electron correlation through the configuration interaction procedure. Phase shifts are also calculated for very low energies, an extremely difficult region for accurate theoretical investigation. The results are found to be in excellent agreement with the most accurate experimental results of O'Malley and Crompton.PACS numbers: 34.80.BmThe scattering length is a very important quantity in order to determine cross sections of electron-atom collisions at thermal energies. At low energies the cross sections in neon vary very rapidly and as a result extrapolations of the experimental data are difficult and cannot be regarded as having produced accurate results. Because of the small scattering length with neon, accurate ab initio calculations are extremely difficult. Moreover, the calculations are made more difficult by the large electron correlations and dynamical polarizations at zero and very low energies. Most accurate methods for electron-atom scattering compute phase shifts in such a way that at zero or very low energies they blow up or give null results. Also at very low energies there is poor convergence and the results are less accurate. Generally the scattering length is determined by computing phase shifts at lower energies with reasonable accuracy and then extrapolating (tan<5o)M to zero energy. However, this does not guarantee the accuracy of the result.There have been a number of determinations 1-8 from experimental measurements and few theoretical calculations 9 " 13 on scattering length with neon. The most accurate determination of scattering length from experimental measurements is due to O'Malley and Crompton. 1 They determined the scattering length and the phase shifts at low energies from an analysis of Robertson's 8 drift-velocity measurements, corresponding to very low swarm energies, through an improved method of analysis employing modified effective-range theory (MERT). 14 15 The MERT is an expansion of the partial-wave phase shifts as a power series in the wave number k with a few coefficients such as the scattering length and one or two others as parameters to be determined by fitting to the best available experimental data. The cross sections are then determined down through the thermal region to zero energy, which is the energy region most difficult to study experimentally. Robertson 8 made drift-velocity measurements in neon at 77 and 300 K and used these data to obtain a momentum-transfer cross section whose accuracy was estimated to be ± 3% for low energies between 0.04 and 6.0 eV. Robertson fitted the experimental values of the momentum-transfer cross section by the modified effective-range formula for energies less than 0.2 eV and obtained the scattering length.As the ab initio calculations ...

The distorted-wave Born approximation has been very successful for treating electron-impact ionization (e,2e) of heavy atoms for high-energy incident electrons. However, as the energy of the incident electrons approaches threshold, significant differences between experiment and theory are observed. In these calculations, the continuum projectile electron wavefunction is typically calculated using the static field of the atom plus a local approximation for electron exchange. While this approximation is believed to be reasonable for higher energies, it is likely to become unreliable for energies near threshold. Here we report a proper treatment of electron exchange in which a full Hartree-Fock calculation is performed for both the atomic and projectile electrons. For the initial state, the projectile orbitals are calculated in the Hartree-Fock approximation with full exchange with the target electrons and for the final state, the Hartree-Fock continuum orbitals are computed for an ion. It is found that the static-exchange approximation is not valid for lower incident energy projectiles.