An improved polynomial approach to the exact perturbative evaluation of low-frequency dynamic polarizabilities for a ground-state hydrogen atom

A recently proposed exact power expansion for the dynamic (frequency-depend ent) multipole polarizabilities of a ground-state hydrogen atom has been im proved keeping its simple polynomial structure substantially unaffected and enlarging its range of convergence. The improvement is performed by adding unexpanded frequency terms taken from the formal expression of the polariz ability as a sum-over-states and subtracting their corresponding power expa nsions. The enlarged range of convergence obtained in this way covers all r eal frequencies of physical importance and the related imaginary frequencie s. (C) 2001 Elsevier Science B.V. All rights reserved.