ON THE CONSTRUCTION OF AN EFFICIENT BASIS FOR CI CALCULATIONS - APPLICATION TO A SPIN FORBIDDEN TRANSITION IN P-II

In the calculation of atomic structure using the configuration interaction (CI) approach, very large numbers of terms may be present in important mixing configurations. This causes computational difficulties in, for example, the non-relativistic calculation of low-lying terms in the transition elements which may be serious enough to prohibit or seriously limit investigation of these ions. A method is presented which allows one to proceed in a sequence of progressively more complicated CI calculations while retaining only a relatively small number of terms with little attendant loss in accuracy. The method is demonstrated in a calculation on the simpler ion, P II. Values for the lifetime for the 3s3p3 5S2o level are obtained both in a full relativistic calculation and in a relativisitic calculation based on a 'progressive reduction of terms' in preliminary non-relativistic calculations. Sources and magnitudes of errors in the calculation are discussed. The full calculation result of 178 mus for the lifetime of this level is in good agreement with a recent experimental value of 167 +/- 12 mus.