Are There Atoms Inside Atomic Anions? H and Li Revisited.

Yes, atoms are still there. We show that the inner electrons of H and Li behave almost as if the outer electrons were not present and that the effect of electron correlation is such that the outer loosely bound electron tries to stay as close as possible to the nucleus but without interfering too much with the electronic distribution of the inner electrons in such a way that the electronic densities of H and Li are still recognizable inside the respective negative ions. We introduce the concept of partial electronic distribution, one for each electron: a particular decomposition of the electronic density.

On the nature of the two-positron bond: evidence for a novel bond type.

The nature of the newly proposed two-positron bond in (PsH), which is composed of two protons, four electrons and two positrons, is considered in this contribution. The study is done at the multi-component-Hartree-Fock (MC-HF) and the Diffusion Monte Carlo (DMC) levels of theory by comparing data, analyzing the spatial structure of the DMC wavefunction, and applying the multi-component quantum theory of atoms in molecules and the two-component interacting quantum atoms energy partitioning schemes to the MC-HF wavefunction. The analysis demonstrates that (PsH) to a good approximation may be conceived of as two slightly perturbed PsH atoms, bonded through a two-positron bond.

e(PsH): A three-positron molecule with a positronic chemical bond.

Two new positronic molecules have been recently discovered: eH [Charry et al., Angew. Chem.

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

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) 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.

The stability of eH .

The recently discovered positronic molecule eH [J. Charry et al., Angew.

Nodal surfaces and interdimensional degeneracies.

The aim of this paper is to shed light on the topology and properties of the nodes (i.e., the zeros of the wave function) in electronic systems.

The structure of the asymmetric helium trimer (3)He(4)He2.

Despite their apparent simplicity, the properties of the two helium trimers, (4)He3 and (3)He(4)He2, are still not completely understood. In particular, the existence of a bound state of the asymmetric trimer (3)He(4)He2 was established many years ago, using different theoretical approaches, and later it was experimentally detected. However its structural properties have not been thoroughly investigated so far, probably because an accurate theoretical description of this very weakly bound system is computationally quite demanding.

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.

Generalized variational principle for excited states using nodes of trial functions.

The familiar variational principle provides an upper bound to the ground-state energy of a given Hamiltonian. This allows one to optimize a trial wave function by minimizing the expectation value of the energy. This approach is also trivially generalized to excited states, so that given a trial wave function of a certain symmetry, one can compute an upper bound to the lowest-energy level of that symmetry.

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 (-34.

What is the shape of the helium trimer? A comparison with the neon and argon trimers.

Despite its apparent simplicity and extensive theoretical investigations, the issue of what is the shape of the helium trimer is still debated in the literature. After reviewing previous conflicting interpretations of computational studies, we introduce the angle-angle distribution function as a tool to discuss in a simple way the shape of any trimer. We compute this function along with many different geometrical distributions using variational and diffusion Monte Carlo methods.

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, Li(2) at the equilibrium geometry, a multideterminantal generalized valence bond wave function does not improve the nodal surfaces of a restricted Hartree-Fock wave function.

Improved diffusion Monte Carlo propagators for bosonic systems using Itô calculus.

The construction of importance sampled diffusion Monte Carlo (DMC) schemes accurate to second order in the time step is discussed. A central aspect in obtaining efficient second order schemes is the numerical solution of the stochastic differential equation (SDE) associated with the Fokker-Plank equation responsible for the importance sampling procedure. In this work, stochastic predictor-corrector schemes solving the SDE and consistent with Itô calculus are used in DMC simulations of helium clusters.

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.

Unexpected symmetry in the nodal structure of the He atom.

The nodes of even simple wave functions are largely unexplored. Motivated by their importance to quantum simulations of fermionic systems, we have found unexpected symmetries in the nodes of several atoms and molecules. Here, we report on helium.

Delayed rejection variational Monte Carlo.

An acceleration algorithm to address the problem of multiple time scales in variational Monte Carlo simulations is presented. After a first attempted move has been rejected, the delayed rejection algorithm attempts a second move with a smaller time step, so that even moves of the core electrons can be accepted. Results on Be and Ne atoms as test cases are presented.

Stability of (3)He(2)(4)He(n) and (3)He(3)(4)He(n) L=0 clusters.

We have studied the stability of mixed (3)He/(4)He clusters in L=0 states by the diffusion Monte Carlo method, employing the Tang-Toennies-Yiu He-He potential. The clusters (3)He(4)He(N) and (3)He(2)(4)He(N) are stable for N>1. The lighter atoms tend to move to the surface of the cluster.