Exotic behavior and crystal structures of calcium under pressure

Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc \rightarrow bcc \rightarrow simple cubic \rightarrow Ca-IV \rightarrow Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is similar to that for Sr. The {\beta}-tin (I41/amd) structure, rather than simple cubic, is predicted to be the theoretical ground state at 0 K and 33-71 GPa. This structure can be represented as a large distortion of the simple cubic structure, just as the higher-pressure phases stable between 71 and 134 GPa. The structure of Ca-V, stable above 134 GPa, is a complex host-guest structure. According to our calculations, the predicted phases are superconductors with Tc increasing under pressure and reaching ~20 K at 120 GPa, in good agreement with experiment

Aggregation effects in proton collisions with water dimers

Charge transfer cross sections in proton collisions with water dimers are calculated using an ab initio method based on molecular orbitals of the system. Results are compared with their counterpart in proton-water collisions to gauge the importance of intermolecular interactions in the cross sectionsThis work has been supported by the project ENE2007-62934 of the Secretaría de Estado de Investigación, Desarrollo e Innovación (Spain). Allocation of computational time at the CCC of the Universidad Autónoma de Madrid is gratefully acknowledge

Charge exchange in proton collisions with the water dimer

We calculate the electron capture cross sections in collisions of protons with water dimers, using a simple ab initio approach. The formalism involves one-electron scattering wave functions and a statistical interpretation to evaluate many-particle cross sections. By comparing with proton-water collisions, we aim at identifying aggregation effects in the electron capture cross section

Ionization of water molecules by proton impact: Two nonperturbative studies of the electron-emission spectra

Two nonperturbative methods are applied to obtain total and singly differential (in the electron energy) cross sections of electron emission in proton collisions with H2O at impact energies in the range 10 keV ≤ Ep ≤ 5 MeV. Both methods, one classical and one semiclassical, combine an independent particle treatment with a multicenter model potential description of the target. The excellent agreement obtained with experimental data supports the usefulness of the approximations involved and encourages the study of more complex systemsThis work has been partially supported by Projects No. ENE2007-62934 and No. ENE2011-28200 (Secretaría de Estado de I+D+i, Spain

Ab initio treatment of ion-water molecule collisions with a three-center pseudo potential

We calculate electron capture cross sections in collisions of protons with water molecules, using two simple ab initio approaches. The formalism involves the calculation of one-electron scattering wave functions and the use of three-center pseudo potential to represent the electron H2O+ interaction. Several methods to obtain many-electron cross sections are considere

Ab initio calculation of charge transfer in proton collisions with N 2

Total and partial charge transfer cross sections are calculated in collisions of protons with the nitrogen molecule at energies between 0.1 and 10 keV. Ab initio potential energy curves and nonadiabatic couplings have been obtained for a number of N2 bond lengths using a multireference configuration interaction method. The influence of the anisotropy of the target molecule is investigated. Results are compared with previous experimental and theoretical dataThis work has been supported by the Project ENE2007- 62934 of the Ministerio de Ciencia e Innovación (Spain). Allocation of computational time at the CCC of the Universidad Autónoma de Madrid is gratefully acknowledge

A numerical lattice approach for ionization and capture processes in ion-H2O collisions

We present a semi-classical numerical integration method for studying electron loss processes in ion collisions with water molecules. Capture and ionization cross section are calculated in the intermediate-high energy range, and the kinetic energy distribution of the ejected electrons is analyzed. The results are compared with those obtained with a classical Monte Carlo metho

Ab initio treatment of charge transfer in ion-molecule collisions based on one-electron wave functions

Two simple ab initio methods based on one-electron wave functions are employed to calculate the singleelectron capture and single ionization of H2O and CO molecules by ion impact. The anisotropy of the molecular targets is taken into account by using multicenter pseudopotentials to represent the interaction of the active electron with the ionic molecular core. These two methods are applied to the study of three collisional systems: H+ + H2O, He2+ + H2O, and C2+ + CO. Comparison with experiments and other theoretical works is presented when availableThis work has been supported by DGICYT Project No. ENE2007-62934/FTN and by AIHH-HH2006-006-ESP- 40/200

Ion-water collisions at intermediate energies

We employ the independent event electron model in the framework of the classical trajectory Monte Carlo method to compute ionization and capture cross sections for one- and two- electron processes in collisions of H+, He2+ and C6+ with water molecules at intermediate impact energies. Subsequente fragmentation processes are also considered, as well as the differential cross sections for electron emission, of paramount importance for ion beam therap

State-selective electron capture in collisions of ground and metastable N2+ ions with H(1s)

9 págs.; 12 figs.; 1 tab.; PACS number(s): 34.70.1e, 34.10.1xA calculation of the electron capture (EC) cross sections for collisions of metastable and ground states of nitrogen2+ ions with H(1s) was presented. The double translational energy spectroscopy technique facilitated the energy change spectrum in EC to be measured for an incident pure beam of ground state ions. It was found that the nuclear wave functions were derived by solving numerically the system of differential equations. It was observed that for impact energies 1 KeV, the impact parameter method was employed, where the nuclei followed straightline trajectories with constant relative velocity V and impact parameter b (R=b+vt). ©2004 The American Physical SocietyI.R. is grateful to the Spanish MCyT for a “Contrato Ramón y Cajal.” This work has been partially supported by DGICYT Projects No. BFM2000-0025 and FTN2000-0911.Peer Reviewe