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The externally contracted CI method applied to N 2
Author(s) -
Siegbahn Per E. M.
Publication year - 1983
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.560230602
Subject(s) - atomic physics , dissociation (chemistry) , basis set , chemistry , bond dissociation energy , perturbation theory (quantum mechanics) , excitation , molecule , perturbation (astronomy) , convergence (economics) , atom (system on chip) , physics , quantum mechanics , computer science , economics , embedded system , economic growth
An approximate multireference CI method is presented. By grouping together configurations with the same internal parts and freezing their relative weights by the use of perturbation theory, the number of variational parameters is drastically reduced. The loss of correlation energy is shown to be usually less than 2%, and the timing is less than one ordinary CI iteration. Examples from calculations on some states of the nitrogen atom and nitrogen molecule are given. The basis set convergence for the lowest excitation energy in the atom is very slow. Less than 50% of the correlation effect is obtained at the s , p , d limit. After the inclusion of ƒ functions this value is improved to 83%. The dissociation energies of the molecule also show slow basis set convergence with errors of 0.5 eV even after addition of ƒ functions. The bond distances are, howeever, accurately reproduced with errors of less than 0.005 Å for all the states. A qualitative discussion of predissociation in the a 1 Π g and B 3 Π g states caused by spin–orbit interaction with the 5 Σ g + state, is finally presented. Rapidly oscillating lifetimes between the different vibrational states are predicted.