1 GGA-PBE*GGA-PBE

This line tells NRLMOL which functional you want it to use. the text before the asterisk is for the exchange and after the asterisk is the correlation. The functional used here, GGA-PBE, is the recommended functional to use. This recommended functional is the generalised gradient approximation functional of Perdew, Burke and Ernzerhof. It is possible to use different functionals for the exchange and correlation, but you would want to be very sure that you had a good reason for doing so^3.1. Functionals available to be used are

• LDA-NONE: none. Probably not all that useful for calculations.
• LDA-PERZUN: Perdew-Zunger 81 [10]
• LDA-CEPERL: Ceperley
• LDA-RPA: RPA
• LDA-WIGNER: Wigner
• LDA-GUNLUN: Gunnarsson-Lundqvist
• LDA-PW91: Perdew-Wang 91 (Local Density form)
• GGA-PW91: Perdew-Wang 91 (Generalised Gradient form)
• LDA-PBE: Perdew-Burke-Ernzerhof 96 [11] (Local Density form)
• GGA-PBE: Perdew-Burke-Ernzerhof 96 [11] (Generalised Gradient form). The recommended functional to use.

find references for all of these. perhaps at http://www.cse.clrc.ac.uk/qcg/dft/references.html The functionals that start with LDA use the Local Density Approximation, while those starting with GGA use the Generalised Gradient Approximation.