The array names in FEFF are a bit cryptic due to the six character limit in standard FORTRAN -- the comments given in 'dim.h' and elsewhere in the source code explain what the names mean. If you need to run larger problems than the dimension statements in the code allow, simply change the dimensions in all the relevant parameter statements and recompile. However, dimensions associated with the atom and phase shift codes, e.g. the number of r-mesh points and the max number of angular momentum levels should NOT be changed.
You can change the main code dimensions in the file 'dim.h' to adjust the code for your computer or for your problem. If you want or need to modify the FEFF code, please contact the authors. No part of FEFF can be included in other codes without a license or permission from the authors. Our development version with all the comments (and subroutines in individual files!) is also available, although it makes use of some features that are not standard FORTRAN. We do not guarantee that any new version will be compatible at the code level with any changes you may make.