[Wien] Single precision corruption, cross platforms/compilers ?

[John Pask]

Dear Wien developers/users,

I have found different total energies for the same calculation, depending 
on compilation options. Which raises (at least) the question, which is 
right? The total energies are different on the order of 0.2 mRy per atom 
depending on compilation options. 

I installed the latest version (WIEN2k_05.2) and compiled with all default
parameters (in siteconfig), then added the -r8 option to extend precision
of default reals/complexes (as is frequently done to rectify inadvertent
single precision constants/declarations). I checked different compilers
(intel and Compaq) and platforms (Linux PC and Compaq unix alpha) and
found consistent differences in all cases between total energies produced
by r8-compiled codes and default-compiled codes.

For example, I found the following total energies for W in various 
structures on Linux PC with ifort 8, mkl 7:

      defaults      -r8           diff
bcc  -32332.244753 -32332.244935  0.000182
tri  -32332.126852 -32332.127035  0.000183
tri2 -64664.490420 -64664.490756  0.000336

[bcc = bcc structure, tri = 1-atom triclinic, tri2 = 2-atom triclinic]

The above runs were executed using run_lapw -ec .000001 -in1new 3 -i 60 to
achieve thorough SCF convergence in all cases (which was verified). For
each structure, the default and -r8 calculations were executed starting
from the same input files (==> same XC, RKmax, Gmax, k-points, etc.). The
only difference was the compilation options used to create the
executables.

Obviously, there are number of implications for an error/uncertainty of 
this order. Any suggestions as to the cause(s) and remedies, and which (if 
either) answer is correct would be most appreciated. (I have appended some 
of the relevant bcc input files below, in case they are of interest.)

Thanks for your help,

John

--------------------------------------
Dr. John E. Pask
Lawrence Livermore National Laboratory
University of California
P.O. Box 808, L-045
Livermore, CA 94551 USA

========================== bcc files ======================
.struct file:
bcc
B   LATTICE,NONEQUIV.ATOMS:  1
MODE OF CALC=RELA unit=bohr
  5.970000  5.970000  5.970000 90.000000 90.000000 90.000000
ATOM   1: X=0.00000000 Y=0.00000000 Z=0.00000000
          MULT= 1          ISPLIT= 2
W          NPT=  781  R0=0.00000500 RMT=    2.5000   Z: 74.0
LOCAL ROT MATRIX:    1.0000000 0.0000000 0.0000000
                     0.0000000 1.0000000 0.0000000
                     0.0000000 0.0000000 1.0000000
  48      NUMBER OF SYMMETRY OPERATIONS
 1 0 0 0.0000000
 0-1 0 0.0000000
 0 0-1 0.0000000
[ rest of 48 ops omitted ....]

.in0 file:
TOT   13     (5...CA-LDA, 13...PBE-GGA, 14...PW2-GGA)
NR2V      (R2V)

.in1 file:
WFFIL        (WFPRI, SUPWF)
  7.00       10    4 (R-MT*K-MAX; MAX L IN WF, V-NMT
 .66740   7   0      global e-param with N other choices, napw
 0    0.565     0.000 CONT 1
 0   -4.439     0.000 CONT 1
 1    0.738     0.000 CONT 1
 1   -1.815     0.000 CONT 1
 2    0.775     0.000 CONT 1
 3    0.825     0.000 CONT 1
 3   -1.077     0.000 CONT 1
K-VECTORS FROM UNIT:4   -7.0       1.5      emin/emax window

.in2 file:
TOT             (TOT,FOR,QTL,EFG,FERMI)
      -9.0      28.0 0.50 0.05                EMIN, NE, ESEPERMIN, ESEPER0
TETRA    0.000          (GAUSS,ROOT,TEMP,TETRA,ALL      eval)
  0 0  4 0  4 4  6 0  6 4
 14.          GMAX
NOFILE        FILE/NOFILE  write recprlist

.inm file:
BROYD  0.0   YES  (BROYD/PRATT, extra charge (+1 for additional e), norm)
0.10            mixing FACTOR for BROYD/PRATT scheme
0.50  1.50      PW and CLM-scaling factors