Sturmians

The method of many-electron Sturmian basis functions is applied to molecnles. The basis potential is chosen to be the attractive Conlomb potential of the nnclei in the molecnle. When such basis functions are used, the kinetic energy term vanishes from the many-electron secular equation, the matrix representation of the nnclear attraction potential is diagonal, the Slater exponents are automatically optimized, convergence is rapid, and a solution to the many-electron Schrodinger eqeuation, including correlation, is obtained directly, without the use ofthe self-consistent field approximation. 

It is interesting to examine the momentum-space orthonormality relations of the Fourier transforms of hydrogenlike Sturmian basis sets. If we let... 

The momentum-space orthonormality relation for hydrogenlike Sturmian basis sets, equation) 17), can be shown to be closely related to the orthonormality relation for hyperspherical harmonics in a 4-dimensional space. This relationship follows from the results of Fock [5], who was able to solve the Schrodinger equation for the hydrogen atom in reciprocal space by projecting 3-dimensional p-space onto the surface of a 4-dimensional hypersphere with the mapping ... 

Many-Particle Sturmians Applied to Molecules dQe ulhi uj) = J kj(s)hi(sj) (3g)... 

Goscinski s treatment of the orthonormality relations of Sturmian basis sets [3] is easy to generalize and we can see by an argument analogous to equations (9) and (10) that when P ,... 

Sturmian basis set obeys a potential-weighted orthogonality relationship analogous to equation (10). This still does not tell us how to normalize the functions, and in fact the choice is arbitrary. However, it will be convenient to choose the normalization in such a way that in momentum space the orthonormality relations become ... 

The definition of the matrix in equation (60) requires some explanation The minus sign is motivated by the fact that H(x) is assumed to be an attractive potential. Division by Po is motivated by the fact that for Coulomb systems, when is so defined, it turns out to be independent of po, as we shall see below. The Sturmian secular equation (61) has several remarkable features In the first place, the kinetic energy has vanished Secondly, the roots are not energy values but values of the parameter po, which is related to the electronic energy of the system by equation (52). Finally, as we shall see below, the basis functions depend on pq, and therefore they are not known until solution... 

In order to illustrate the discussion given above, let us consider the simplest possible example - the H2 molecule with a basis consisting of a single two-electron Sturmian ... 

The results presented in this paper seem to indicate that it will be possible to apply successfully the method of many-electron Sturmians to molecules. Momentum-space methods, pioneered by Shibuya and Wulfman [7], seem very well suited to solving the one-electron part of the problem. When the basis potential used in constructing the many-electron Sturmian basis set is taken to be the nuclear attraction potential experienced by the electrons in the molecule, the method of many-electron Sturmians has the following advantages ... 

Basis sets of the type discussed in this paper can only be applied to bound-state problems. It is interesting to ask whether it might be possible to constmct many-electron Sturmian basis sets appropriate for problems in reactive scattering in an analogous way, using hydrogenlike continuum functions as building-blocks. We hope to explore this question in future publications. 

Since the hydrogenlike Sturmian basis functions form a complete set, the term %i,o,o(xy-R) can be represented as a single-center expansion in terms of functions localized at the origin ... 

The relationship between alternative separable solutions of the Coulomb problem in momentum space is exploited in order to obtain hydrogenic orbitals which are of interest for Sturmian expansions of use in atomic and molecular structure calculations and for the description of atoms in fields. In view of their usefulness in problems where a direction in space is privileged, as when atoms are in an electric or magnetic field, we refer to these sets as to the Stark and Zeeman bases, as an alternative to the usual spherical basis, set. Fock s projection onto the surface of a sphere in the four dimensional hyperspace allows us to establish the connections of the momentum space wave functions with hyperspherical harmonics. Its generalization to higher spaces permits to build up multielectronic and multicenter orbitals. 

This paper considers the hyperspherical harmonics of the four dimensional rotation group 0(4) in the same spirit ofprevious investigations [2,11]), where the possibility is considered of exploiting different parametrizations of the 5" hypersphere to build up alternative Sturmian [12] basis sets. Their symmetry and completeness properties make them in fact adapt to solve quantum mechanical problems where the hyperspherical symmetry of the kinetic energy operator is broken by the interaction potential, but the corresponding perturbation matrix elements can be worked out explicitly, as in the case of Coulomb interactions (see Section 3). A final discussion is given in Section 4. 

The Sturmian eigenfunctions in momentum space in spherical coordinates are, apart from a weight factor, a standard hyperspherical harmonic, as can be seen in the famous Fock treatment of the hydrogen atom in which the tridimensional space is projected onto the 3-sphere, i.e. a hypersphere embedded in a four dimensional space. The essentials of Fock analysis of relevance here are briefly sketched now. 

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