Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:‚Ä¶and complex nature, the PCM model with RB3LYP or RHF method has been used to simplify the optimization of some compounds in the GAUSSIAN 98 program (Becke, 1992a, 1992b, 1993, 1996; Foresman & Frisch, 1996; Frisch & Foresman, 1998; Hehre et al., 1986; Larid, Ross, & Ziegler, 1996; Miertus &‚Ä¶
Category: