Positron Binding to Lithium Excited States

In the last 15 years hundreds of papers have been devoted to the study of positron-atom or positron-molecule interaction. A large body of evidence has accumulated showing that many atoms in their ground state can bind a positron forming an electronically stable system. Studies on the possibility that a positron binds to an atomic excited state, however, are scarce. The first atom that was proved able to bind a positron in its ground state is lithium. Surprisingly, nothing is known on the possibility that a positron could bind to one of its excited states. In this Letter we study the positron attachment to the 1s22p 2Po, 1s2s2p 2Po and 2p3 4So excited states of the lithium atom. While the 2Po state cannot bind a positron, and the 4So could at most form a metastable state, a positron can attach to the 4Po state of lithium forming a bound state with a binding energy of about 0.003 hartree. This state can alternatively be considered an excited state of the system e+Li and it could be, in principle, exploited in an experiment to detect e+Li, whose existence has been predicted theoretically but has not yet been observed experimentally

Two positrons can form a chemical bond in (PsH)2

We show that two positrons can form a chemical bond between two otherwise repelling ions, similar to what happens to two hydrogen atoms forming a hydrogen molecule. Two positronium hydride atoms (PsH) can form the stable species (PsH)2 when the two coupled positrons have opposite spins, while they form an antibonding state if they have the same spin. This is completely analogous to the landmark description by Heitler and London [Z. Phys. 44, 455 (1927)] on the formation of a chemical bond in the hydrogen molecule coupling two electrons with opposite spins. This is the first time two positrons are shown to behave like two electrons in ordinary matter, enlarging the definition of what is a chemical bond dating back to Lewis [J. Am. Chem. Soc. 38, 762 (1916)]. We suggest a few experimental routes to form and detect such a peculiar molecule

Quantum Monte Carlo study of the H- impurity in small helium clusters

We report ground state energies and structural properties for small helium clusters (4He) containing an H- impurity computed by means of variational and diffusion Monte Carlo methods. Except for 4He_2H- that has a noticeable contribution from collinear geometries where the H- impurity lies between the two 4He atoms, our results show that our 4He_NH- clusters have a compact 4He_N subsystem that binds the H- impurity on its surface. The results for $N\geq 3$ can be interpreted invoking the different features of the minima of the He-He and He-H- interaction potentials.Comment: 12 pages, 7 Ps figure

On the nodal structure of single-particle approximation based atomic wave functions

The nodal structures of atomic wave functions based on a product of spatial orbitals, namely, restricted, unrestricted, and generalized valence bond wave functions, are shown to be equivalent. This result is verified by fixed node-diffusion Monte Carlo simulations for atoms up to Ne. Also for a molecular system, Li2 at the equilibrium geometry, a multideterminantal generalized valence bond wave function does not improve the nodal surfaces of a restricted Hartree-Fock wave function

Stability and production of positron-diatomic molecule complexes

The energies at geometries close to the equilibrium for the e(+)LiF and e(+)BeO ground states were computed by means of diffusion Monte Carlo simulations. These results allow us to predict the equilibrium geometries and the vibrational frequencies for these exotic systems, and to discuss their stability with respect to the various dissociation channels. Since the adiabatic positron affinities were found to be smaller than the dissociation energies for both complexes, we propose these two molecules as possible candidates in the challenge to produce and detect stable positron-molecule systems. Moreover, low-energy positron scattering on LiF and BeO targets may show vibrational Feshbach resonances as fingerprints of the existence of stable ground states of e(+)LiF and e(+)BeO

Quantum Monte Carlo calculations of the dimerization energy of borane

Accurate thermodynamic data are required to improve the performance of chemical hydrides that are potential hydrogen storage materials. Boron compounds are among the most interesting candidates. However, different experimental measurements of the borane dimerization energy resulted in a rather wide range ( 1234.3 to 1239.1) \ub1 2 kcal/mol. Diffusion Monte Carlo (DMC) simulations usually recover more than 95% of the correlation energy, so energy differences rely less on error cancellation than other methods. DMC energies of BH3, B2H6, BH3 CO, CO, and BH2+ allowed us to predict the borane dimerization energy, both via the direct process and indirect processes such as the dissociation of BH3CO. Our De = 1243.12(8) kcal/mol, corrected for the zero point energy evaluated by considering the anharmonic contributions, results in a borane dimerization energy of 1236.59(8) kcal/mol. The process via the dissociation of BH3CO gives 1234.5(2) kcal/mol. Overall, our values suggest a slightly less De than the most recent W4 estimate De = 1244.47 kcal/mol [A. Karton and J. M. L. Martin, J. Phys. Chem. A 111, 5936 2007)]. Our results show that reliable thermochemical data for boranes can be predicted by fixed node (FN)-DMC calculations

An investigation of nodal structures and the construction of trial wave functions

The factors influencing the quality of the nodal surfaces, namely, the atomic basis set, the single-particle orbitals, and the configurations included in the wave-function expansion, are examined for a few atomic and molecular systems. The following empirical rules are found: the atomic basis set must be fairly large, complete active space and natural orbitals are usually better than Hartree-Fock orbitals, multiconfiguration expansions perform better than single-determinant wave functions, but only few configurations are effective and their choice is suggested by symmetry considerations, while too long determinantal expansions spoil the nodal surfaces. These rules allow us to reduce the nodal error and to compute the best fixed node-diffusion Monte Carlo energies for a series of dimers of first-row atoms