The CORREL command reads a number of trajectory files whose Fortran unit numbers are specified sequentially. The first unit is given by the FIRSTU keyword and must be specified. NUNIT gives the number of units to be scanned, and defaults to 1.
BEGIN, STOP, and SKIP are used to specify which steps in the trajectory are actually used. BEGIN specifies the first step number to be used. STOP specifies the last. SKIP is used to select steps periodically as follows: only those steps whose step number is evenly divisible by STEP are selected. The default value for BEGIN is the first step in the trajectory; for STOP, it is the last step in the trajectory; and for SKIP, the default is 1.
Reorienting a coordinate trajectory is possible using the COMPARE command. For details, Reorienting a Coordinate Trajectory.