Partitioning scheme for density functional calculations of extended systems.

We show that, at least for the ground electronic state of systems treated using semilocal density functionals (like in local density or generalized gradient approximations), a calculation of the entire extended nonperiodic system consisting of several well distinguished parts (e.g., a collection of molecules) can be replaced with a finite set of calculations on specifically chosen smaller subsystems that overlap with each other. Every subsystem is terminated with link (or pseudo) atoms (or groups of atoms) to reduce the effect of the termination. However, because of the particular choice of the subsystems, the effect of the link atoms is largely compensated in the final total energy if the subsystems are chosen sufficiently large. In fact, we prove that the proposed method should result in nearly the same total energy, electronic density and atomic forces as a single (considered as a reference) density functional calculation on the entire system. Our method, however, should be much more efficient due to unfavorable scaling of the modern electronic structure methods with the system size. The method is illustrated on examples of serine water, lysine-water and lysine dimer systems. We also discuss possible approximate applications of our method for quantum-classical calculations of extended systems, when, as compared to widely used quantum-mechanical/molecular-mechanical methods, the problem of the quantum cluster boundary can be eliminated to a large degree.

Microsolvation of Li(+) in Small He Clusters. Li(+)Hen Species from Classical and Quantum Calculations.

A structural study of the smaller Li(+)Hen clusters with n ≤ 30 has been carried out using different theoretical methods. The structures and the energetics of the clusters have been obtained using both classical energy minimization methods and quantum Diffusion Monte Carlo. The total interaction acting within the clusters has been obtained as a sum of pairwise potentials: Li(+)-He and He-He. This approximation had been shown in our earlier study to give substantially correct results for energies and geometries once compared to full ab initio calculations. The general features of the spatial structures, and their energetics, are discussed in details for the clusters up to n = 30, and the first solvation shell is shown to be essentially completed by the first 8-10 helium atoms.

Role of boson-fermion statistics on the Raman spectra of Br2(X) in helium clusters.

The role played by the bosonic or fermionic character of He atoms surrounding a Br2(X) molecule is analyzed through vibrotational Raman spectra simulations. Quantum chemistry-type calculations reveal the spin multiplicity to be chiefly responsible for the drastic difference observed by Grebenev et al. [Science 279, 2083 (1998)]] in the rotational structure of molecules embedded in helium droplets.

Raman spectra of (He)N-Br2(X) clusters: the role of boson/fermion statistics in a quantum solvent.

The aim of this paper is to elucidate the role played by the bosonic/fermionic character of N He atoms solvating a Br2(X) molecule. To this end, an adiabatic model in the molecular stretching coordinate is assumed and the ground energy levels of the complexes are searched by means of Hartree (or Hartree-Fock) Quantum Chemistry calculations for 4He (or 3He) solvent atoms. Simulations of vib-rotational Raman spectra point at the spin multiplicity as the main feature responsible for the drastic difference in the rotational structures of molecules embedded in boson or fermion helium drops as already observed by the experiments of Grebenev et al. [S. Grebenev, J. P. Toennies, and A. F. Vilesov, Science 279 (1998) 2083].

Replacement equivalence of H- and argon in small (Ar)nH- clusters from optimized structure calculations.

The structural properties of some of the smaller ionic clusters of argon atoms containing the atomic impurity H-, ArnH- with n from 2 up to 7, are examined using different modeling for the interactions within each cluster and by employing different theoretical treatments, both classical and quantum, for the energetics. The same calculations are also carried out for the corresponding neutral homogeneous clusters Ar(n+1). The results of the calculations, the physical reliability of the interactions modeling, and the similarities and the difference between the anionic and the neutral complexes are discussed in some detail. The emerging picture shows that, due to specific features of the employed atom-atom potentials, the ArnH- and Ar(n+1) clusters present very similar structures, where the H- dopant substitutes for one of the outer Ar atoms but does not undergo as yet solvation within such small clusters.

Quantitative immunochemistry of salivary proteins adsorbed in vitro to enamel and cementum from caries-resistant and caries-susceptible human adults.

Salivary proteins adsorbed to powdered enamel and cementum from parotid and submandibular saliva (1:1) of caries-resistant (CR) and caries-susceptible (CS) subjects were examined qualitatively by polyacrylamide disc electrophoresis and quantitatively by immunochemical procedures. The electrophoretic patterns showed no consistent difference between enamel and cementum or between CR and CS samples. Quantitatively, there were no significant differences between CR and CS samples, but significant differences between enamel and cementum. On the basis of ng/mm2, enamel adsorbed five times as much acidic proline-rich protein and cysteine-containing protein as cementum and nearly twice as much lysozyme. There were no significant differences in adsorption of albumin and lactoferrin. The lack of difference in the concentration of pellicle proteins in CR and CS subjects may be related to similar findings with parotid and submandibular saliva. The influence of the major differences in surface pellicle proteins between enamel and cementum on the properties of their respective pellicles is uncertain.