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The stability of the ground and excited states of Positronium-atom complexes ͓A, Ps͔, Ps ϭ͓e ϩ ,e Ϫ ͔, has been explored for AϭLi, B, C, O, F using variational and diffusion Monte Carlo techniques. From the numerical results of our simulations it turns out that the ground state of the complexes ͓Li, Ps͔ 2 S, ͓C, Ps͔ 3 S, ͓O, Ps͔ 1 P, and ͓F, Ps͔ 2 S is stable against the break up in the two neutral fragments A and Ps, while the ground state of ͓B, Ps͔ 2 P has an energy above the same dissociation threshold. As to the excited states, the only possible candidate, ͓F, Ps͔ 2 P, has a total energy statistically equal to the lower dissociation threshold, i.e. it does not seem to be stable against the dissociation.

We present a quantum Monte Carlo study of the structure and energetics of silver doped helium clusters AgHen for n up to 100. Our simulations show the first solvation shell of the Ag atom to be composed by roughly 20 He atoms, and to possess a structured angular distribution. Moreover, the electronic 2 P 1/2 ← 2 S 1/2 and 2 P 3/2 ← 2 S 1/2 electronic transitions of the embedded silver impurity have been studied as a function of the number of helium atoms. The computed spectra show a redshift for n ≤ 15 and an increasing blueshift for larger clusters, a feature attributed to the effect of the second solvation shell of He atoms. For the largest cluster, the computed excitation spectrum is found in excellent agreement with the ones recorded in superfluid He clusters and bulk. No signature of the direct formation of proposed AgHe2 exciplex is present in the computed spectra of AgHe100. PACS numbers: Superfluid4 He clusters represent a gentle environment where high resolution spectroscopic studies of atoms, atomic clusters, and molecules at low temperature can be carried out [1]. In such cold and fluid quantum systems many perturbing effects due to the temperature and solid matrices are absent, allowing therefore for an easier interpretation of the experimentally recorded spectra. Moreover, their superfluid behavior allows interesting quantum effects to take place and to be experimentally probed (for instance see Refs. [2,3]).Whereas the coupling of the rotational and vibrational motion of the molecules with the quantum motion of the solvent is permitted by the similarity between energy levels, the electronic structure of an atom is characterized by energy differences orders of magnitude larger than the ones needed to induce excitation in the atomic motion. Although this difference might seem to work in the direction of simplifying the physical description of the electronic transition processes, many important details still wait to be clarified. As an example, the fluorescent D 2 emission line (i.e. the 2 S 1/2 ← 2 P 3/2 radiative transition) of heavy single valence electron atoms dispersed in superfluid helium is absent, while the D 1 line is sharp and only slightly shifted (1-2 nm) to the blue [4]. This is in contrast with the large broadening and strong blueshift of the absorption lines. Moreover, some features of the LIF spectra of the dispersed Ag were interpreted as signature of the AgHe and AgHe 2 exciplex formation [5].The blueshift and broadening of the absorption lines * Electronic address: Massimo.Mella@unimi.it † Present address: Laboratory of Inorganic Chemistry, ETH Hönggenberg, CH-8093 Zürich, Switzerland; Electronic address: Colombo@inorg.chem.ethz.ch ‡ Electronic address: Gabriele.Morosi@uninsubria.it have been interpreted by means of a "bubble model". Here, the dispersed atom is enclosed in a spherical cavity due to the exchange repulsion of its valence electrons and the He ones. The liquid He around an atom is modeled by an isotropic sharp-edge density profile with no atomic internal structure. How...

The stability of the ground-state of positron-polar molecule complexes ͓M,e ϩ ͔ has been explored for MϭLiH,HF,H 2 O,BeO,LiF using variational and diffusion Monte Carlo techniques. Our simulations show that the ground-state of the complexes ͓LiH,e ϩ ͔ 2,1 ⌺ ϩ , ͓BeO,e ϩ ͔ 2,1 ⌺ ϩ , and ͓LiF,e ϩ ͔ 2,1 ⌺ ϩ is stable against the dissociation either in the two fragments M and e ϩ or in the other two fragments M ϩ and Psϭ͓e ϩ ,e Ϫ ͔, while the ground-state of ͓H 2 O,e ϩ ͔ 2,1 A 1 , and of ͓HF,e ϩ ͔ 2,1 ⌺ ϩ has an energy equal to the dissociation threshold, M and e ϩ. We also compare the predicted vertical positron affinity ͑PA͒ with high quality vertical electron affinity ͑EA͒ and discuss the relevant difference between the two values.

The energy variance optimization algorithm over a fixed ensemble of configurations in variational Monte Carlo often encounters problems of convergence. Being formally identical to a problem of fitting data, we re-examine it from a statistical maximum-likelihood point of view. We show that the assumption of an underlying Gaussian distribution of the local energy, implicit in the standard variance minimization scheme, is not theoretically nor practically justified, and frequently generates convergence problems. We propose alternative procedures for optimization of trial wave functions in quantum Monte Carlo and successfully test them by optimizing a trial wave function for the helium trimer

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Structure and energetics of small helium clusters: Quantum simulations using a recent perturbational pair potential Small helium ( 4 He) clusters containing the lighter isotope 3 He are studied by means of quantum Monte Carlo methods. Accurate ground state energies and structural properties are obtained using accurate trial wave functions and the Tang-Tonnies-Yiu ͑TTY͒ helium-helium pair potential. The dimer 4 He-3 He is not bound; as well as the trimer 4 He 3 He 2 . The smallest cluster containing 3 He is 4 He 2 3 He with a nonrigid structure having a marked linear contribution. Interestingly, this weakly bound system, with an energy one order of magnitude less than the 4 He 3 trimer, is able to bind another 3 He atom, forming the tetramer 4 He 2 3 He 2 , which shows the odd feature of having five out of six unbound pairs. In general, the substitution of a single 4 He atom in a pure cluster with a 3 He atom leads to an energetic destabilization, as the pair 4 He-3 He is not bound. The isotopic impurity is found to perturb only weakly the distributions of the remaining 4 He atoms, which retain the high floppiness already found in the pure clusters. As the number of atoms increases the isotopic impurity has the marked tendency to stay on the surface of the cluster. This behavior is consistent with the formation of the so-called ''Andreev states'' of a single 3 He in liquid 4 He helium and droplets, where the impurity tends to form single-particle states on the surface of the pure 4 He.

We discuss the accuracy and applicability to molecular systems of the model proposed by G. G. Ryzhikh and J. Mitroy [J. Phys. B 31, 5013 (1998), ibid. 32, 2203 (1999)] to compute annihilation rate in positron-containing systems. Moreover, we propose small modifications to this model in order to improve its accuracy for molecular systems.