12 Trouble shooting

In this chapter hints are given for solving some difficulties that have occurred frequently. This chapter is by no means complete and the authors would appreciate further suggestions which might be useful for other users. Beside the printed version of the users guide, an online pdf version is available using help_lapw. You can search for a specific keyword (use $^{\wedge}$f keyword) and hopefully find some information.

If an error occurs in one of the SCF programs, a file program.error is created and an error message is printed into these files. The run_lapw script checks for these files and will automatically stop if a non-empty error file occurs.

Check the files case.dayfile (which is written by init_lapw and run_lapw), :log (where a history of all commands using x is given) and *.error for possible explanations.

In several places the dimensions are checked. The programs print a descriptive error message and stop.

case.outputnn:

This file gives error messages if the atomic spheres overlap. But it should also be used to check the distances between the atoms and the coordination number (same distance). If inconsistencies exists, your case.struct file may contain an error. A check for overlapping spheres is also included in mixer and lapw1.

case.outputd:

Possible stops or warnings are:

``NO SYMMETRY OPERATION FOUND IN ROTDEF``:

This indicates that in your case.struct file either the positions of equivalent atoms are not specified correctly (only positive coordinates allowed!!) or the symmetry operations are wrong.

case.output1:

Possible stops or warnings are:

``NO ENERGY LIMITS FOUND IN SELECT``:

This indicates that $E_{top}$ or $E_{bottom}$ could not be found for some $u_l(r,E_l)$. Check your input if it happens in the zeroth iteration. Later, (usually in the second to sixth iteration) it may indicate that in your SCF cycle something went wrong and you are using a crazy potential. Usually it means that mixing of the charge densities was diverging and large charge fluctuations occured. Check previous charges for being physically reasonable (grep for labels :NTOxx :CTOxx :DIS :NEC01). Usually this happens when your input is not ok, or for very ill conditioned problems (very rare), or more likely, when ``Ghostbands'' appeared (or some states were missing) because of bad energy parameters in case.in1. You will probably have to delete case.broy* and case.scf, rerun x dstart and then change some calculational parameters. These could be: fixing some energy parameter (modify both, case.in1 and case.in1_orig or try the -in1orig switch if you have used -in1new); switch to a broadening method (TEMP with eg. 0.010 mRy); or increase the k-mesh (magnetic metals); or reduce the mixing parameter in case.inm slightly (eg. to 0.1). In very difficult (magnetic) cases a PRATT mixing with eg. 0.01 mixing might be helpful at the beginning of the scf cycle (but later switch to MSEC1 again) !

``STOP RDC_22``:

This indicates that the overlap matrix is not positive definite. This usually happens if your case.struct file has some error in the structure or if you have an (almost) linear dependent basis, which can happen for large RKMAX values and/or if you are using very different (extremely small and large) sphere radii $R_{MT}$.

``X EIGENVALUES BELOW THE ENERGY emin``:

This indicates that X eigenvalues were found below emin. Emin is set in case.in1 (see sec. 7.3.3) or in case.klist generated by KGEN, see 6.3, 6.5). It may indicate that your value of emin is too high or the possibility of ghostbands, but it can also be intentional to truncate some of the low lying eigenvalues.

If you don't find enough eigenvalues (e.g.: in a cell with 4 oxygens you expect 4 oxygen s bands at roughly -1 Ry) check the energy window (given at the end of the first k-point in case.in1 or in case.klist) and make sure your energy parameters are found by subroutine SELECT or set them by hand at a reasonable value.

case.output2:

Possible stops or warnings are:

``CANNOT BE FOUND``:

This warning, which could produce a very long output file, indicates that some reciprocal K-vector would be requested (through the k-vector list of lapw1), but was not present in the list of the K generated in lapw2. You may have to increase the NWAV, and/or KMAXx parameters in lapw2 or increase GMAX in case.in2. The problems could also arise from wrong symmetry operations or a wrong structure in case.struct.

``QTL-B VALUE``:

If larger than a few percent, this indicates the appearance of ghost bands, which are discussed below in section 12.1.
The few percent message (e.g up to 10 %) does not indicate a ghost band, but can happen e.g. in narrow d-bands, where the linearization reaches its limits. In these cases one can add a local orbital to improve the flexibility of the basis set. (Put one energy near the top and the other near the bottom of the valence band, see section 7.3.3).

FERMI LEVEL not converged

(or similar messages). This can have several reasons: i) Try a different Fermi-Method (change TETRA to GAUSS or TEMP in case.in2). ii) Count the number of eigenvalues in case.output1 and compare it with the number of valence electrons. If there are too few eigenvalues, either increase EMAX in case.klist (from 1.5 to e.g. 2.5) or check if your scf cycle had large charge oszillations (see SELECT error above)

If the SCF cycle stops somewhere (especially in the first few iterations), it is quite possible, that the source of the error is actually in a previous part of the cycle or even in a previous (e.g. the zeroth) iteration. Check in the case.scf file previous charges, eigenvalues, ...whether they are physically reasonable (see SELECT error above).

1 Ghost bands

Approximate linear dependence of the basis set or the linearization of the energy dependence of the radial wave functions (see section 2.2) can lead to spurious eigenvalues, termed ``ghost bands''.

The first case may occur in a system which has atoms with very different atomic sphere radii. Suppose you calculate a hydroxide with very short O-H bonds so that you select small $R_{MT}$ radii for O and H such as e.g. 1.0 and 0.6 a.u., respectively. The cation may be large and thus you could choose a large $R_{MT}$ of e.g. 2.4 a.u. However, this gives a four time larger effective RKmax for the cation than for H, (e.g. 16.0 when you select RKmax=4.0 in case.in1). This enormous difference in the convergence may lead to unphysical eigenvalues. In such cases choose lmax=12 in case.in1 (in order to get a very good re-expansion of the plane waves) and reduce $R_{MT}$ for the cation to e.g. 1.8 a.u.

The second case can occur when you don't use a proper set of local orbitals. In this situation the energy region of interest (valence bands) falls about midway between two states with different principle quantum numbers, but with the same l-value (for one atom).

Take for example Ti with its $3p$ states being occupied as (semi-core) states, while the $4p$ states remain mostly unoccupied. In the valence band region neither of those two states (Ti $3p$, $4p$) should appear. If one uses 0.2 Ry for the expansion energy E(1) for the $p$ states of Ti, then Ti-p states do appear as ghost bands. Such a run is shown below for $TiO_2$ (rutile).

The lowest six eigenvalues at GAMMA fall between about -1.30 and -1.28 Ry. They are ghost bands derived from fictitious Ti-p states. The next four eigenvalues between -0.94 and -0.78 Ry correspond to states derived from O 2s states, which are ok, since there are four O's per unit cell, four states are found.

The occurrence of such unphysical (indeed, unchemical!) ghostbands is the first warning that something went wrong. A more definite warning comes upon running LAPW2, where the corresponding charge densities are calculated. If the contribution to the charge density from the energy derivative of the basis function [the $B_{lm}$ coefficient in equ. 2.4,2.7] is significant (i.e. much more than 5 per cent) then a warning is issued in LAPW2.

In the present example it reads:

QTL-B VALUE .EQ. 40.35396 !!!!!!

This message is found in both the case.scf file and in case.output2.

When such a message appears, one can also look at the partial charges (QTL), which are printed under these conditions to OUTPUT2, and always appear in the files case.helpXXX, etc., where the last digit refers to the atomic index.

In the file below, note the E(1) energy parameter as well as the 6 ghost band energies around -1.29.

--------------- top of file:tio2.scf -----------------------------
          ATOMIC SPHERE DEPENDENT PARAMETERS FOR ATOM  Titanium 
          OVERALL ENERGY PARAMETER IS     .2000
          E( 0)=     .2000
--->      E( 1)=     .2000
          E( 2)=     .2000   E(BOTTOM)=    -.140   E(TOP)= -200.000


          ATOMIC SPHERE DEPENDENT PARAMETERS FOR ATOM  Oxygen 
          OVERALL ENERGY PARAMETER IS     .2000
          E( 0)=    -.7100   E(BOTTOM)=   -2.090   E(TOP)=     .670

       K=    .00000    .00000    .00000            1
:RKM  : MATRIX SIZE= 599  RKM= 6.99  WEIGHT= 8.00  PGR:    
       EIGENVALUES ARE:
        -1.2970782   -1.2970782   -1.2948747   -1.2897193  -1.2897193
        -1.2882306    -.9389111    -.8484857    -.7880729   -.7880729
         -.0484830    -.0162982     .0121181     .0976534    .0976534
          .1914068     .1914068     .2341991     .3286919    .3477629
          .3477629     .3809219     .5143729     .5356211    .5550735
          .5617155     .5617155     .7087550     .7197110    .8736991
          .8736991     .9428865     .9533619    1.2224570   1.2224570
         1.4285169
       ********************************************************
       NUMBER OF K-POINTS:          1 

:NOE  : NUMBER OF ELECTRONS          =  48.000
:FER  : F E R M I - ENERGY           =    .53562

:POS01: AT.NR. -1  POSITION =  .00000  .00000  .00000  MULTIPLICITY=  2
       LMMAX=10
       LM=  0 0 2 0 2 2 4 0 4 2 4 4 6 0 6 2 6 4 6 6 0 0 0 0 0 0 0 0 0 0 0 0 0 0
:CHA01: TOTAL CHARGE INSIDE SPHERE   1 =     8.802166
:PCS01: PARTIAL CHARGES SPHERE =  1 S,P,D,F,PX,PY,PZ,D-Z2,D-X2Y2,D-XY,D-XZ,D-YZ 
:QTL01:  .127 6.080 2.518  .067 2.011 2.047 2.022 1.090  .760 .155  .480  .034
                      VXX         VYY         VZZ       UP TO R
:VZZ01:            -4.96856     8.48379    -3.51524       2.000
:POS02: AT.NR. -2  POSITION =  .30500  .30500  .00000  MULTIPLICITY=  4
       LMMAX=16
       LM=  0 0 1 0 2 0 2 2 3 0 3 2 4 0 4 2 4 4 5 0 5 2 5 4 6 0 6 2 6 4 6 6 0 0
:CHA02: TOTAL CHARGE INSIDE SPHERE   2 =     5.486185
:PCS02: PARTIAL CHARGES SPHERE =  2 S,P,D,F,PX,PY,PZ,D-Z2,D-X2Y2,D-XY,D-XZ,D-YZ 
:QTL02: 1.559 3.902  .022  .002 1.296 1.306 1.300  .014  .004 .000  .003  .001
                      VXX         VYY         VZZ       UP TO R
:VZZ02:              .25199     -.55091      .29892       1.600

:CHA  : TOTAL CHARGE INSIDE CELL =      48.000000
:SUM  : SUM OF EIGENVALUES =            -15.810906

   QTL-B VALUE .EQ.   40.35396   !!!!!!
      NBAND in QTL-file:         24 
----------------end of truncated file tio2.scf----------------------

Next we show tio2.output2 for the first of the ghost bands at -1.297 Ry. One sees that it corresponds mainly to a p-like charge, which originates from the energy derivative part Q(UE) of the Kohn-Sham orbital. Q(UE) contributes 40.1% compared with 8.5% from the main component Q(U). Q(UE) greater than Q(U) is a good indication for a ghost band.

----------------part of file tio2.output2 --------------------------
   QTL-B VALUE .EQ.   40.35396   !!!!!!
  K-POINT:   .0000   .0000   .0000   599  36           1
  BAND #  1  E= -1.29708  WEIGHT= 2.0000000
           L= 0     L= 1       PX:      PY:      PZ:    L= 2    DZ2:   DX2Y2:     DXY:     DXZ:     DYZ:    L= 3  
 QINSID:    .0000  48.6035  35.0996  13.5039    .0000    .0000    .0000    .0000    .0000    .0000    .0000    .0030
 Q(U)  :    .0000   8.4902   6.0125   2.4777    .0000    .0000    .0000    .0000    .0000    .0000    .0000    .0026
 Q(UE) :    .0000  40.1132  29.0871  11.0261    .0000    .0000    .0000    .0000    .0000    .0000    .0000    .0005
           L= 0     L= 1       PX:      PY:      PZ:    L= 2      DZ2:   DX2Y2:     DXY:     DXZ:     DYZ:    L= 3  
 QINSID:    .1294    .0707    .0000    .0055    .0653    .0088    .0038    .0049    .0000    .0000    .0000    .0022
 Q(U)  :    .1279    .0627    .0000    .0052    .0575    .0087    .0038    .0049    .0000    .0000    .0000    .0020
 Q(UE) :    .0016    .0081    .0000    .0003    .0077    .0001    .0000    .0000    .0000    .0000    .0000    .0002
 QOUT  : 1.9265
----------------------bottom of truncated file ----------------------

Another file in which the same information can be found is tio2.help031, since the ghost band is caused by a bad choice for the Ti-p energy parameter:

----------------------Top of file tio2.help031 ---------------------
  K-POINT:   .0000   .0000   .0000   599  36           1
  BAND #  1  E= -1.29708  WEIGHT= 2.0000000
  L= 0     .00000      .00000    .00000    .00000    .00000    .00000
  L= 1   48.60346     8.49022  40.11324    .00000    .00000    .00000
    PX:  35.09960     6.01247  29.08712    .00000    .00000    .00000
    PY:  13.50386     2.47774  11.02612    .00000    .00000    .00000
    PZ:    .00000      .00000    .00000    .00000    .00000    .00000
  L= 2     .00000      .00000    .00000    .00000    .00000    .00000
   DZ2:    .00000      .00000    .00000    .00000    .00000    .00000
 DX2Y2:    .00000      .00000    .00000    .00000    .00000    .00000
   DXY:    .00000      .00000    .00000    .00000    .00000    .00000
   DXZ:    .00000      .00000    .00000    .00000    .00000    .00000
   DYZ:    .00000      .00000    .00000    .00000    .00000    .00000
  L= 3     .00304      .00255    .00050    .00000    .00000    .00000
  L= 4     .00000      .00000    .00000    .00000    .00000    .00000
  L= 5     .00096      .00082    .00014    .00000    .00000    .00000
  L= 6     .00000      .00000    .00000    .00000    .00000    .00000
-------------------bottom of truncated file--------------------------

Note again for L=1 the percentage of charge associated with the primary (APW) basis functions ul (8.5%) versus that coming from the energy derivative component (40.1%).

If a ghost band appears, one should first analyze its origin as indicated above, then use appropriate local orbitals to improve the calculation and get rid of these unphysical states.

Do not perform calculations with ``ghost-bands'', even when the calculation converges.

Good luck !