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ABSTRACT:Optimal Gaussian-type orbital GTO basis sets of positron and electron in positron-molecule complexes are proposed by using the full variational treatment of Ž . molecular orbital FVMO method. The analytical expression for the energy gradient with respect to parameters of positronic and electronic GTO such as the orbital exponents, the Ž . orbital centers, and the linear combination of atomic orbital LCAO coefficients, is derived. Wave functions obtained by the FVMO method include the effect of electronic or positronic orbital relaxation explicitly and satisfy the virial and Hellmann᎐Feynman theorems completely. We have demonstrated the optimization of each orbital exponent in Ž . various positron-atomic and anion systems, and estimated the positron affinity PA as the difference between their energies. Our PA obtained with small basis set is in good agreement with the numerical Hartree᎐Fock result. We have calculated the OH y and w y q x OH ; e species as the positron-molecular system by the FVMO method. This result shows that the positronic basis set not only becomes more diffuse but also moves toward the oxygen atom. Moreover, we have applied this method to determine both the nuclear and electronic wave functions of LiH and LiD molecules simultaneously, and obtained the isotopic effect directly.

The binding energy of positronium chloride is calculated using a model potential for the ten core electrons and the quantum Monte Carlo method for the eight valence electrons and the positron. The result is 1.91 ±0.16 eV. Except for three-and four-particle systems, this is the first accurate calculation of the binding energy of a compound containing a positron. PACS numbers: 36.10.Dr, 14.60.Cd, 31.20.Di, 82.55.+e Compounds containing a positron in addition to electrons and nuclei are important in several areas: surface studies, ceramic and doped Cw superconductors, radiation chemistry, many-body quantum mechanics, voids in polymers and molecular crystals, mass spectrometry (especially of biologically significant compounds), etc. [ll. In spite of their importance, our knowledge of their binding energies is extremely sparse. No direct measurements have been reported, although very recent progress has been made [2]. Except for the present work, accurate quantum mechanical calculations have been applied only to two-, three-, and four-particle systems. Positronium chloride (PsCl) is an atom consisting of a chlorine atom combined with a positronium atom. It is stable compared to separated CI and Ps atoms. Its ACAR (angular correlation of annihilation radiation) spectrum has been observed in aqueous solutions of chloride ions [3-5], graphite-intercalated chloride compounds [6,7], and chloride-doped polyacetylene [8]. The observed ACAR curves agree closely with that calculated from the Hartree-Fock wave function for gaseous PsCl [9] (although adding waters of hydration to the calculation degrades the agreement somewhat [10]). None of this experimental work gives an indication of the magnitude of the binding energy-just its sign. An assertion has been made that the PAL (positron annihilation lifetime) spectrum of chlorine gas and of argon-chlorine gas mixtures gives the Ps-Cl bond strength as 2.0 ±0.5 eV [11]. Experimental evidence supporting this assertion was not given [11,12]. Theoretical evidence that PsCl is bound was provided in 1953 by Simons, who calculated a positronic orbital in the fixed field of a chloride ion represented by a HartreeFock wave function [13]. Simons' calculated Ps-Cl bond energy was 0.59 eV, which is a lower bound. This calculation was repeated some years later with a more modern chloride wave function, with similar results [14]. Cade and Farazdel added full self-consistency, which increased the calculated bond energy slightly to 0.73 eV [15,16]. These calculations omit two important sources of stability for the system: polarization and correlation. The first is a long-range effect and can be adequately treated in a very simple way by including a polarization potential term in the Hartree-Fock equation for the positron.Correlation is a short-range effect and is much more difficult to treat by conventional quantum mechanical techniques. The purely electronic correlation energy amounts to about 20 eV [17]. The PsCl binding energies quoted in this paragraph are compromised because they rest on the...

A method previously used by the authors in an accurate calculation of the binding energy of positronium chloride [Phys. Rev. Lett. 68, 3281 (1992)] is applied to positronium fluoride and positronium bromide. The binding energies obtained with this method are PsF, 1.98±0.17 eV; PsCl, 1.91±0.16 eV; PsBr, 1.14±0.11 eV.

Ab initio calculations are made to examine theoretically the possibility of stable existence of [OH−;e+] system. Diffuse functions are added to the conventional 6-31G basis set, considering the wide spread of positron orbital. Moreover, the Mo/ller–Plesset perturbation of the second order is calculated to take the electron correlation into account. These two improvements are found to be very effective for the stable existence of the system. The positron affinity of OH− is computed to be 4.9 eV, and the binding energy of positronium to OH as 0.7 eV which is in good agreement with experimental estimate.