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Numerical studies for a theoretical analysis of semiempirical LCAO – CI methods
Author(s) -
Carpentieri M.,
Porro L.,
Del Re G.
Publication year - 1968
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560020608
Subject(s) - hamiltonian (control theory) , linear combination of atomic orbitals , physics , chemistry , atomic orbital , electron , statistical physics , computational chemistry , quantum mechanics , mathematics , mathematical optimization
A numerical investigation of Del Re and Parr's formulas [1] for the treatment of π systems has been preformed in the case of five‐membered rings, using two different expressions for the core Hamiltonian and different values for the effective charges. The results obtained are discussed by analysing the three stages of the calculation: ( a ) a non‐iterative LCAO – MO calculation; ( b ) the same calculation with corrections for exchange and repulsion terms arising from fluctuations of the orbital populations; ( c ) configuration interaction. The calculations are interesting also because they do not involve the zero differential overlap approximation; a calculation without inclusion of overlap hse been carried out for pyrrole and the results have been compared with those including S . The main conclusions hold also for σ electrons, and can serve to assess better the validity of simple σ calculations.