You can do a partial inertia relief calculation using the PSOLVE command (PSOLVE is described in Part II: Archived Commands). Use the partial solution method as shown in the command input below:

/PREP7
 ...
 ...
MP,DENS,...         ! Generate model, define density
 ...
 ...
FINISH

/SOLU
D,...               ! Specify only minimum number of constraints
F,...               ! Other loads
SF,...
OUTPR,ALL,ALL       ! Activates printout of all items
IRLF,1              ! Can also be set to -1 for precise mass and
                    !  load summary only, no inertia relief
PSOLVE,ELFORM       ! Calculates element matrices and inertia relief terms
IRLIST              ! Lists the mass summary and total load summary tables
FINISH

See the OUTPR, IRLF, and IRLIST commands in the Command Reference. See also the PSOLVE command in Part II: Archived Commands.