Jorge E. Pérez

Analysis of the diamagnetic spin-orbital contribution to indirect nuclear spin-spin coupling constants

Abstract A general numerical method for the calculation of the diamagnetic spin-orbital (DSO) contribution to the nuclear spin-spin coupling is presented. Cylindrical coordinates are used to carry out the integration of the perturbative Hamiltonian in order to overcome the singularities which are present at the sites of the coupled nuclei. Advantages and shortcomings of the present method with respect to others already published are discussed. The decomposition of the DSO coupling in terms of localized orbitais describing bonds or lone pairs is also proposed in order to predict in simple terms which bonds or lone pairs yield important contributions to the coupling. This method is thus particularly useful in the analysis of the “through-space” transmission of the DSO interaction.

2J(SeSe) couplings in diseleno-substituted alkenylic compounds. A CLOPPA-IPPP analysis

Abstract A CLOPPA-IPPP analysis of 2 J (SeSe) couplings in two cyclic diseleno-substituted alkenylic compounds is performed. The total FC, PSO, DSO, and SD components of these couplings are evaluated within the RPA-INDO approximation, and total values are compared to the experimental values. The PSO interaction yields large values in both cases. Particularly, it is the dominant interaction in the cyclic plane compound. For this case, an analysis of the main transmission mechanisms of the SO term is performed in terms of localized molecular orbitals (LMOs) representing chemical functions. It is seen that the coupling is mainly transmitted by the occupied and vacant LMOs associated with the Se atoms. The influence of the π-type Se lone pairs is negligible, while the inplane Se lone pairs play an important role. Additivity of pathways is also analyzed: a “direct” and an “indirect” nonadditivity of paths are defined. A high direct nonadditivity is observed. A curious influence of the occupied π vinylic MO in the indirect nonadditivity is shown. Even though it is known that in several geminal couplings the DSO term yields an important contribution, it has a value of only about 10 −2 Hz in the present case.

Numerical evaluation of three‐ and four‐center bielectronic integrals using exponential‐type atomic orbitals

Three‐ and four‐center Slater orbital bielectronic integrals are evaluated by means of a complete function set. The method provides a series to approximate the bielectronic integrals. Their corresponding partial sums are analyzed in detail for 1s orbitals. The comparison with the Fourier transform–based method brings forth encouraging perspectives for the present approach.

Implementation of atomic basis set composed of 1s Gaussian and 1s Slater‐type orbitals to carry out quantum mechanics molecular calculations

A mixed atomic basis set formed with ls Slater‐type orbitals and 1s floating spherical Gaussian orbitals is implemented. Evaluation of multicenter integrals is carried out using a method based on expansion of binary products of atomic basis functions in terms of a complete basis set, and a systematic analysis is performed. The proposed algorithm is very stable and furnishes fairly good results for total energy and geometry. An LCAO‐SCF test calculation is carried out on LiH. The trends observed show that there are some combinations of mixed orbitals that are appropriate to describe the system. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 604–609, 1999

On the Evaluation of Four‐Center Two‐Electron Integrals involving 1s Slater Type Orbitals

The four‐center two‐electron integral (12|34) involving 1s Slater Type Orbitals satisfies (12|34) = F(Q), for a unique Q and an appropriated function F(Q). Based on this relation, an algorithm is designed to calculate a sequence which permits the approximation of (12|34). The examples considered show very good convergence for the sequence; this fact encourages further studies concerning fast algorithms of evaluation to get reliable approximants of (12|34) with this method.