Re: Selenium basis set ( No.1 ) 
 Date: 2007/02/20 19:48
 Name: Vasilii.Artyukhov
 You can use any basis set starting from s1p1. This one will have one s function and three p functions ('px', 'py', and 'pz', although this designation has not much of a meaning for a free atom). Then, in a free atom, the wavefunctions of two electrons would be described by the slike function, and the other four would be described by three plikes. Adding more basis functions is needed to allow some radial flexibility (and angular, if you add dfunctions), similarly to the multiplezeta concept of Gaussian basis sets. So, the answer to your question is, your basis must contain at least one s and one p function, but you would perhaps want to add more of them and even some d functions to improve your results. The idea is to check whether an additional d function gives you more benefit than an additional sp function pair. For instance, s1p1d1 seems to be better than s2p2. An remember that there's a very nice orbital optimization feature, so you can optimize an s5p5d5 basis to something like s2p2d1 and have very nice results at a moderate price.

