CCL:G: How to do Double 3d Shell Calculation

["lizhe3(a)mail.ustc.edu.cn" <lizhe3(a)mail.ustc.edu.cn>]

 Sent to CCL by: "lizhe3-*-mail.ustc.edu.cn"
 [lizhe3-*-mail.ustc.edu.cn]
 Dear All,
 I read in literature that the 3d shell of transition metals has a
 correlation effect called "3d double shell effect"(eg. K Andersson, BO
 Roos. Chem Phys Letters 191:507–514, 1992.and TH Dunning Jr, BH Botch,
 JF Harrison. J Chem Phys 72:3419–3420, 1980.). They described the way to
 address this problem by adding a second more diffuse d shell to the
 electron configuration in CASSCF calculation, eg. the original
 configuration of Cu atom is 3d(10)4s(1) can be transformed to
 3d(9)3d'(1)4s(1), with 3d' a more diffuse shell. But I do not know how
 to do this practically. It seems in the CASSCF method of Gaussian
 software I can only include the 3d orbital. Should I also include the 4d
 orbital or just use more diffuse basis set? Does anyone know how to add
 the 3d' orbital?