If you have access to the NAG libraries on the platform on which you
wish to run these codes, you can replace the inversion routine with
calls to the equivalent NAG routine. This can be done by uncommenting
lines 2093-2095 in CASTEP/overlap.F90 and 2723-2725 in
CETEP/cetep_overlap.f90 and commenting out lines 2101 and 2731
respectively. The OVERLAP_LIBS line must then be commented out
in the Makefiles for your architecture.
The NAG routine should be better optimised for your architecture than the code included in the overlap distribution.