Models potential from nuclear charge

Theoretical considerations leading to a density functional theory (DFT) formulation of the reaction field (RF) approach to solvent effects are discussed. The first model is based upon isolelectronic processes that take place at the nucleus of the host system. The energy variations are derived from the nuclear transition state (ZTS) model. The solvation energy is expressed in terms of the electrostatic potential at the nucleus of a pseudo atom having a fractional nuclear charge. This procedure avoids the introduction of arbitrary ionic radii in the calculation of insertion energy, since all integrations involved are performed over [O.ooJ The quality of the approximations made are discussed within the frame of the Kohn-Sham formulation of density functional theory. 

In this section we consider those applications of nuclear charge density distributions which are common practice in electronic structure calculations. We start with the electrostatic potentials resulting from the use of the most popular nuclear models, and continue with standard applications in electronic structure calculations. In the following we consider four popular nuclear models,... 

For finite nucleus models with a well-defined nuclear size parameter R, beyond which the nuclear charge density is exactly( ) zero and the nuclear potential is exactly( ) given as —Z/r (r > R), the energy shifts can be obtained directly from the matching condition for the logarithmic derivatives L (r) = / r)/P [r) of the radial functions in the inner... 

In this work a simple analytical atomic density model is obtained from the expression of a modified Thomas-Fermi-Dirac model with quantum corrections near the nucleus as the minimization of a semiexplicit density functional. The use of a simple exponential analytical form for the density outside the near-nucleus region and the resolution of a single-particle Schrodinger equation with an effective potential near the origin allows us to solve easily the problem and obtain an asymptotic expression for the energy of an atom or ion in terms of the nuclear charge Z and the number of electrons N. 

We have already seen above that the choice of a point-like atomic nucleus limits the Dirac theory to atoms with a nuclear charge number Z < c, i.e., Zmax 137. A nonsingular electron-nucleus potential energy operator allows us to overcome this limit if an atomic nucleus of finite size is used. In relativistic electronic structure calculations on atoms — and thus also for calculations on molecules — it turned out that the effect of different finite-nucleus models on the total energy is comparable but distinct from the energy of a point-like nucleus (compare also section 9.8.4). 

We approach the effect of finite nuclear charge models from a formal perspective and introduce a general electron-nucleus potential energy Vnuc, which may be expanded in terms of a Taylor series around the origin. 

If a nuclear charge distribution with a finite radius is to be used, the question of the functional form of the distribution must be raised. The point nuclear model was simple now we have to consider some form of the nuclear charge distribution that bears some relation to experimentally determined distributions. The Coulomb potential at a point r from a charge distribution p (r) is... 

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