The generalized separated electron pair model. 1. An application to NH₃

Abstract

AbstractsThe utility of the separated electron pair (SEP) model (strongly orthogonal geminals) is examined quantitatively, for pyramidal and planar nuclear configurations of the NH, molecule. The best SEP wave function computed for each species is capable of recovering about half of the correlation energy obtained by a fairly accurate configuration interaction (GI) calculation, (corresponding to roughly 2 5 O b of the total molecular correlation energy). I t is illustrated that the model can be systematically extended with only a modest effort to yield more accurate results (about 409, of the total correlation energy). The fact that the corrections to the SEP model have a simple physical interpretation suggests that this model may be a useful starting point for "brute force" CI calculations on larger chemical systems. O n examine d u point de vue quantitatif, I'utilitt d u modele des paires stpartes (SEP)pour les configurations nucltaires pyramidales et planes de la moltcule NH, . Avec la meilleure fonction d'onde SEP on retrouve B prts peu la moitit de l'tnergie d e corrtlation obtenue par un calcul d'interaction des configurations (CI) assez prCcis (correspondant B environ 2 5 O " de l'tnergie de corrtlation totale de la moltcule). O n niontre par une illustration que ce modtle peut &tre gtntralist d'une facon systtmatique avec un effort modeste, pour rendre des resultats plus prtcis (environ 40; ;de I'tnergie de correlation totale). Le fait que les corrections au modele SEP posstdent une interpretation physique simple suggere que ce modde-ci peut servir conime point de dtpart pour des calculs GI trts grands pour des systtmes chimiques plus grands.Die Brauchbarkeit des Modells mit getrennten Elektronenpaare (stark orthogonale Geminale) wird in quantitativer Weise fur pyramidale und ebene Kernkonfigurationen des NH,-Molekuls untersucht. Mit der besten sEP-b'ellenfunktion erhalt man ungefahr die Halfte der Korrelationsenergie, die man mit einer recht prazisen Konfigurationswechselwirkungsberechnung (GI) berechnen kann (ungefahr 2500 der totalen molekularen Korrelationsenergie). Es wird illustriert, dass das Modell in systematischer Weise verallgemeinert werden kann um prazisere Resultate (ungefahr 40" , , der totalen Korrelationsenergie) zu geben. Die Tatsache dass die Korrekturen des sEp-Modells eine einfache physikalische Bedeutung haben schlagt vor dass dieses Modell ein nutzlicher Ausgangspunkt ftir "brute force" cI-Berechnungen fur grossere chemische Systeme sein kann.

Characterization of charge distribution in terms of localized orbitals

“…The transformation matrices of AO basis to CMO and CMO basis to LMO, respectively, are in good agreement with earlier investigations [12,13,14,15]. The calculated coefficients for the AO ~ CMO and CMO ~ LMO matrices on the example of H20 moleeule are given in Table I.…”

Geminal model chemistry II. Perturbative corrections

“…[2][3][4][5][6][7] Sometimes this model is referred to as separated electron pair ͑SEP͒ theory. [8][9][10][11][12] Most prior studies focused on the total amount of correlation energy recovered by the product of strongly orthogonal geminals. It was generally concluded that the model has its limitations, primarily attributable to either lack of intergeminal correlation, 5 strong orthogonality approximation, 13 or both.…”

Theory of lone pairs. IV. Molecular ion hole states of ten‐electron hydrides. Molecular ionization potentials and proton affinities by direct SCF calculations

“…With decreasing proton number on the same central atom the correlation energy is expected to increase slightly. 16 The neglect of relativistic effects is known to cause errors mostly in the core ionization energies. The relativisitic correction, of course, is greater for the neutral species than that for the positive ion, and increases fast with the atomic number.17…”