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By Ramon Carbo, Joseph M. Riera

We dwell in a molecular international, nearly closed shell in nature, and hence Chemistry has been a technology facing closed shell mol­ ecules. even if, the excessive measure of experimental sophistication reached some time past decade has made extra obvious the function of open shell buildings in chemical examine. A parallel phenomenon may be saw within the improvement of SCF idea, the place closed shell molecular calculations at any point of complexity compose the most physique of references which are acquired in Quantum Chemistry at the present time. along with the linkage among experimental and theoretical behaviour, there are, evidently, different purposes that are connected to an absence of molecular open shell calculations. between others, there has been no connec­ tionbetween closed or open shell theoretical remedies. during this demeanour, many computational positive factors utilized by closed shell connoisseurs haven't been prolonged to different computational components. because the paintings of Roothaan in 1960, the open shell molecular panorama has been, the­ oretically, a really closed one. additional improvement of SCF concept, which has ended in an outburst of multiconfigurational systems, has paid no, or very faint, realization to the interconnection among those SCF concept complex positive factors, the open shell framework and closed shell universal perform. a great theoretical aim, in most cases conversing, and specifically inside of SCF conception, may well encompass a process which might be used to unravel a given chemical challenge, in the actual and approx­ imate limits of the theory.

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Hillier and Saunders procedure has been proved convenient in many calculations and it can be used in closed or open shell computational algorithms. 1. The Ordering Principle Although there can be other causes, nonconvergence in SCF calculations is mainly due to the mixing among MO's belonging to the various subsets of the molecular basis, as long as the iterative SCF path is followed. A simple traceback analysis of any reluctant case can make this clear. In the original level shift technique, the main purpose is to separate the eigenvalues of each MO within each subset (for example: the occupied MO and the virtual orbitals), preserving the initial ordering of the orbital eigenvalues so the SCF iterations, through an invariant Aufbau principle, converge to an "a priori" chosen wavefunction.

1I) have been chosen with the same absolute value o. The value of a which has given the best convergence pattern in any case has been a 1. Also, at the * - A12 , self-consistent stage the values of the differences ~ = A21 which measure the degree of hermiticity of the off-diagonal Lagrange multipliers, have been computed to be ~10-6 for a convergence criterion of the eigenvector components with the value of 10- 6 • Table 1. IV collects the exponents, eigenvalues and vectors, total energy, virial coefficient, and i c series.

996705 -6 1. 997197 -51. 196433 1. 66 Ne 7 + Basis set {ls,2s} STO. The figures correspond to the optimal energies, upon exponent variation C11 E (Is) (au) exponents (continued) Configuration 1s22s1(2S). IV Results for the Li isoe1ectronic sequence. -6 0> '" 39 permitted us to study the behaviour of the generalized coupling operators without problems. The ground states of He and Be as well as the Li isoelectronic sequence have been studied for this purpose. He, Li and Be ground state wavefunctions have been reproduced from previous classical calculations within the present formalism, using 3 and 4 STO's basis sets.

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