Warning or Error? BRMIX: very serious problems

[alpinnovianus]

Dear VASP experts,

I would like to ask about the following warning/error messages.

the old and the new charge density differ
old charge density: 1056.71970 new 1058.00000
BRMIX: very serious problems

These messages appear in the stdout.log file pasted below.

Code: Select all

 found WAVECAR, reading the header
  number of k-points has changed, file:    30 present:    70
  trying to continue reading WAVECAR, but it might fail
 WAVECAR: different cutoff or change in lattice found
 INCAR ok, starting setup
 POSCAR ok, starting setup
 FFT: planning ... GRIDC
 FFT: planning ... GRID_SOFT
 FFT: planning ... GRID
 reading WAVECAR
 the WAVECAR file was read successfully
 initial charge from wavefunction
 entering main loop
       N       E                     dE             d eps       ncg     rms          rms(c)
BRMIX: very serious problems
 the old and the new charge density differ
DAV:   1    -0.188067878807E+04   -0.18807E+04   -0.14810E+03260480   0.579E+01BRMIX: very serious problems
 the old and the new charge density differ
 old charge density:  1056.71970 new 1058.00000
BRMIX: very serious problems
 the old and the new charge density differ
 old charge density:  1056.71970 new 1058.00000
BRMIX: very serious problems
 the old and the new charge density differ
    0.212E+01
 old charge density:  1056.71970 new 1058.00000
 old charge density:  1056.71970 new 1058.00000
DAV:   2    -0.189373900306E+04   -0.13060E+02   -0.15575E+02304128   0.488E+01    0.202E+01
DAV:   3    -0.189351943871E+04    0.21956E+00   -0.59889E+01319872   0.393E+01    0.155E+01
DAV:   4    -0.189283924298E+04    0.68020E+00   -0.10735E+01279040   0.212E+01    0.746E+00
DAV:   5    -0.189287420618E+04   -0.34963E-01   -0.48235E+00298496   0.175E+01    0.467E+00
DAV:   6    -0.189273215882E+04    0.14205E+00   -0.13453E+00289152   0.881E+00    0.168E+00
DAV:   7    -0.189275161394E+04   -0.19455E-01   -0.20303E-01272256   0.147E+00    0.104E+00
DAV:   8    -0.189275728902E+04   -0.56751E-02   -0.36899E-02290432   0.841E-01    0.751E-01
DAV:   9    -0.189277000068E+04   -0.12712E-01   -0.61238E-02289280   0.973E-01    0.464E-01
DAV:  10    -0.189277715277E+04   -0.71521E-02   -0.24933E-02285312   0.490E-01    0.404E-01
DAV:  11    -0.189278440457E+04   -0.72518E-02   -0.28675E-02278016   0.522E-01    0.339E-01
DAV:  12    -0.189278935844E+04   -0.49539E-02   -0.27366E-02278400   0.751E-01    0.179E-01
DAV:  13    -0.189279349879E+04   -0.41403E-02   -0.29134E-02286464   0.628E-01    0.875E-02
DAV:  14    -0.189279416243E+04   -0.66364E-03   -0.19410E-03286464   0.120E-01    0.540E-02
DAV:  15    -0.189279487578E+04   -0.71335E-03   -0.12441E-03279808   0.126E-01    0.280E-02
DAV:  16    -0.189279503485E+04   -0.15908E-03   -0.93170E-05289792   0.398E-02    0.202E-02
DAV:  17    -0.189279510262E+04   -0.67762E-04   -0.34152E-05287232   0.228E-02    0.110E-02
DAV:  18    -0.189279501672E+04    0.85897E-04   -0.20183E-05305024   0.154E-02    0.552E-03
DAV:  19    -0.189279501556E+04    0.11613E-05   -0.15441E-06268800   0.388E-03    0.336E-03
DAV:  20    -0.189279500254E+04    0.13018E-04   -0.54803E-07288128   0.266E-03    0.265E-03
DAV:  21    -0.189279500928E+04   -0.67412E-05   -0.40300E-07316544   0.241E-03    0.166E-03
DAV:  22    -0.189279500308E+04    0.62008E-05   -0.18934E-07260480   0.267E-03    0.893E-04
DAV:  23    -0.189279500659E+04   -0.35091E-05   -0.11622E-07218112   0.102E-03    0.676E-04
DAV:  24    -0.189279500482E+04    0.17699E-05   -0.17882E-08143360   0.507E-04    0.412E-04
DAV:  25    -0.189279500372E+04    0.11036E-05   -0.30859E-08143232   0.722E-04    0.381E-04
DAV:  26    -0.189279500423E+04   -0.51540E-06   -0.73946E-09143360   0.230E-04    0.213E-04
DAV:  27    -0.189279500442E+04   -0.19315E-06   -0.23961E-09143232   0.186E-04    0.189E-04
DAV:  28    -0.189279500393E+04    0.49322E-06   -0.29106E-09143360   0.188E-04    0.215E-04
DAV:  29    -0.189279500441E+04   -0.48109E-06   -0.11368E-09143232   0.122E-04    0.108E-04
DAV:  30    -0.189279500479E+04   -0.37361E-06   -0.66789E-10143104   0.105E-04    0.925E-05
DAV:  31    -0.189279500496E+04   -0.17205E-06   -0.37916E-10143232   0.518E-05    0.406E-05
DAV:  32    -0.189279500508E+04   -0.12683E-06   -0.28905E-10143104   0.375E-05    0.413E-05
DAV:  33    -0.189279500498E+04    0.10589E-06   -0.26054E-10142464   0.408E-05    0.167E-05
DAV:  34    -0.189279500493E+04    0.51718E-07   -0.14102E-10142848   0.298E-05    0.201E-05
DAV:  35    -0.189279500493E+04   -0.62864E-08   -0.11080E-10141952   0.189E-05
   1 F= -.18927950E+04 E0= -.18927950E+04  d E =0.000000E+00  mag=    -0.0000
 writing wavefunctions

My questions are:

• If these are 'very serious problems' as printed by the messages, why were the calculations allowed to continue and not killed?

• In my case, the messages only appear in the first few steps, and then the calculations proceeded to converge.
  I think the messages could have arisen from the change in k-points density from the initial wavefunction reading.
  I was reading a WAVECAR from a coarse k-points mesh to a calculation with finer mesh.
  
  Because there are no problems with the subsequent steps, I think these messages only reflect the change in initial conditions.
  After many self-consistent loops were performed as above, the results would have been minimized to at least a local minima, if not the ground state, subject to the threshold parameters set in the INCAR.
  
  Therefore, the resulting state may be okay to use as long as we carefully check whether it is a local or global minimum.
  Is my understanding correct?

Thank you.
Alpin

[andreas.singraber]

Dear Alpin,

as you already suggested this warning message most likely originates from the change from the coarse to the fine k-point mesh. Although the number of k-points differs the output tells us that VASP is still trying to start from the given WAVECAR. Consequently, also the initial charge density is computed from this incomplete WAVECAR. At that point the routine that produces the "very serious problems" message is not aware that it is just dealing with a very "roughly-prepared" initial state. If such a problem would occur later during the electronic optimization there would indeed be serious problems. However, in your case it is probably safe to ignore this message and accept the results (given you check for local or global minimum as you mentioned).

What you could also try is to start with ICHARG = 1 which would create the initial charge density from the CHGCAR file. However, I am not entirely sure if this will solve the issue and I have not tried it.

All the best,
Andreas Singraber