Pseudopotential Portability in the QMC Framework
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PSEUDOPOTENTIAL PORTABILITY IN THE QMC FRAMEWORK
M. Menchi*, A. Bosin*, F. Meloni* and G. B. Bachelet** Cagliari, 09124 Cagliari, Italy
"*Dipartimento di Scienze Fisiche, Universitk di "*Dipartimentodi Fisica, I Universit. di Roma
"La Sapienza", 00185 Roma, Italy
ABSTRACT We have calculated the binding energies of several two-electron pseudoions using the Diffusion Quantum Monte Carlo method. The comparison between our results and the experiment suggests that HSC pseudopotentials are portable from the original single particle theory (DFT-LDA) to the "exact" many body one. Moreover we are able to evaluate the degree of portability of each effective field. INTRODUCTION The Quantum Monte Carlo (QMC) methods are promising candidates for an accurate description of the electron-electron interactions in atoms. In principle they permit to exactly deal with many-electron atoms beyond the limits of the approximate single particle theories as, for example, the Hartree-Fock or the Local Density Approximation (LDA). Up to now, the QMC has been used to calculate binding properties (ionization potentials Vjp and electron. affinities VEA) of atoms lighter than Neon and of light molecules (binding energies, bond lenghts and electron affinities) as H 2 , LiH, Li 2 and H20 [1]. One has to extract small valence energies as differences between enormous total energies and the computational time required to obtain negligible statistical errors is very high and strongly increases with the complexity of the system. For atoms, the dependence of the CPU time on Z, the atomic number, is dramatic [10] [11], and the cause can be traced back to the presence of the core electrons. So, while valence electrons are responsible for the chemical-physical properties of interest, the sampling of the (unimportant) core electrons takes up most of the computational time, representing the real limiting factor. The problem is well understood within the single particle scheme and it has been solved by ab-initio Pseudopotentials [2] and Pseudohamiltonians [3] [4]. They replace the nucleus plus core electrons of the real atom by simple, single-particle operators. These operators are generated within a given approximate theory of the electronelectron interaction and originally conceived for use just in the same framework. The pseudoion construction has no reference to the experimental atomic properties, the only input being the atomic number. We are interested into the physical meaning of these pseudoions and, more specifically, into the conditions under which we could export them from the original single-particle theory into other theoretical and computational frameworks as, for instance, QMC. This property of pseudoions has been named "portability". It's a desirable property since, for the moment, there are no well-established methods to generate valence-only atoms within the QMC framework. We are interested in estimating the portability of Norm Conserving HSC Pseudopotentials [2] (in this work, we stick to tabulated HSC ions [5]) into QMC simulations [6].
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