For both gas-phase and surface kinetics reaction mechanisms, users have the option to provide custom reaction-rate formulations or to actually over-ride the entire species rates-of-production calculations with a user-provided subroutine. These options are described in more detail in Chemkin Input Manual. Briefly, to turn on user-programmed rate routines, the user must include either USRPROG, or USRPROD auxiliary keywords in the Gas-phase Kinetics or Surface Kinetics input file. The USRPROG option is entered after a specific reaction line, and serves to override the rate-of-progress calculation for that particular reaction. The USRPROD option, however, is entered on the REACTION line and signals that a user routine will provide calculation of all species net production rates, regardless of the rest of the kinetics input in the file.

For gas-phase chemistry, the applicable user routines for the USRPROG and USRPROD options are CKUPROG and CKUPROD, respectively. Templates for these subroutines are provide in the FORTRAN file cklib_user_routines.f.

For surface chemistry, the applicable user routines for the USRPROG and USRPROD options are SKUPROG and SKUPROD, respectively. Templates of these subroutines are found in sklib_user_routines.f.