PC GAMESS/Firefly CIS MODULE DOCUMENTATION. CIS group required when CITYP=CIS The CIS method (singly excited CI) is the simplest way to treat excited states. ... The CIS code in the PC GAMESS/Firefly is based on heavily modified sources of the original GAMESS (US) AO-basis CIS code written by Dr. Simon P. Webb (Ref. 3 below). ... The state-tracking feature of the PC GAMESS/Firefly' CIS code can be activated by selecting negative value of istate in the $cis group. ... PC GAMESS/Firefly' NBO code . ...