This is a special warning or advice to all users of Pentium/Alpha platforms.
In the following I assume use of the PGF compiler (other native compilers
for Alpha platforms offer a feature similar to that discussed below ...).
It concerns mainly the optics tool. However, it is also of interest in general.

I have to mention that in the example Makefiles (PGF) I always added an option
"-byteswapio" (which I also use in my own VASP Makefiles - in contrast to the
official version distributed from the server in Vienna). I added this option
for all Linux platforms (it also holds for Alpha platforms) since the Pentium
(and Alpha) platforms use a so-called "little-endian" binary representation
for floating point numbers, whereas practically all other platforms (IBM, HP,
SUN, SGI, IEEE-CRAYs, ...) use a "big-endian" format. Since file OPTIC is a
binary file (as WAVECAR is) there will arise an incompatibility if one tries
to interchange any kind of binary file between "little-endian" and "big-endian"
platforms (since the one format reads the bits from left to the right and the
other from the right to the left - i.e. identical bit patterns are interpreted
completly different!). Therefore I have put "-byteswapio" which converts the
internal "little-endian" format of Pentiums (Alphas) into "big-endian" format
on output to binary files and back on reading. If you do not use "-byteswapio"
for both, VASP and any postprocessing tools, then you have the same "little-
endian"/"big-endian" incompatibility and the numbers on binary files as OPTIC
are completly misinterpreted (getting complete nonsense). Since (for reasons
of portability of the binary files) I do not store INTEGERS (as NKPTS etc.) as
integer numbers but also as floating point numbers (e.g. RNKPTS in optics.F)
these numbers are now also completely wrong (4.5849291934067722E-320 instead
of 10.) - they would be also wrong if I had put them on file as integers (I
prefer 64-bit floating-point only since this is common to all IEEE platforms,
for INTEGER, LOGICAL there is no real standard, it's sometimes 32-bit, it's
sometimes 64-bit ...). This usually results already in an error on startup ...

So, if you use Pentiums (or Alphas) with PGF: Either compile all postprocessing
tools without the option "-byteswapio" (or equivalent options for other native
Alpha compilers). Or even (much!) better: recompile VASP with "-byteswapio" as
well - since this vastly increases portability of files as WAVECAR and OPTIC
also to other platforms as IBM, HP, SUN, SGI, IEEE-CRAYs, ... . It is what I
preferred to do for my VASP version since I work in a rather heterogeneous
environment consisting of Pentium, IBM, HP, and CRAY-T3E platforms and I like
to exchange WAVECAR and OPTIC files among all platforms. Hence, I strongly
recommend to all users to do the same as well - it makes life easier ... ;-).
Anyway, take care of compiling both, VASP and any postprocessing tools, using
consistent (compatible) compiler options - or you will suffer serious trouble.