CESD99 – A new version to represent atomic wave functions in a determinant basis Original Research Article

Determinant expansions of atomic wave functions have several advantages for calculating transition probabilities and cross sections. For instance, such a representation can easily be applied to rather a large number of transition operators and also in order to include rearrangement effects of the bound-state density following the excitation or the decay of an atomic state. In practice, the rearrangement of the electron density is most simply taken into account if the variation of the initial and final state wave functions is carried out separately in studying transition or ionization processes. Of course, such a computational procedure yields sets of one-electron orbitals which are not quite orthogonal to each other and which do require special care in evaluating the (many-electron) transition matrix.For relativistic wave functions of the atomic structure package GRASP92 [F.A. Parpia, C.F. Fischer, I.P. Grant, CPC 92 (1996) 249], a determinant representation of the atomic states is obtained by the CESD97 program [S. Fritzsche, I.P. Grant, CPC 103 (1997) 277]. This program performs a complete expansion of symmetry-adapted functions and has been found useful in a number of applications. For accurate transition probability studies, however, often large wave function expansions of several ten or even hundred thousand determinants are required which cannot be handled by CESD97 due to the demand on memory and computing time. Further difficulties were caused by the tight binding of both GRASP92 and CESD97 to the IBM XL Fortran standard. To overcome these difficulties and to facilitate the transfer to other architectures, CESD97 has entirely been rewritten according to the ANSI standard Fortran 90/95. The new version, CESD99, considerably improves the efficiency of large computations and, now, enables wave function expansions based on several (hundred) thousand determinants. The conversion to Fortran 90/95 is considered as major step to render the CESD program ready to use for systematic investigations of open-shell atoms. CESD99 is part of the RATIP package for the calculation of relativistic atomic transition and ionization properties and will be distributed within this framework [S. Fritzsche, C.F. Fischer, C.Z. Dong, this volume].