Explanation of Warnings and Error Messages

FAQ on warnings and errors:

• WANNIER CODE WARNING: GRADIENT FOR RESTA FUNCTIONAL
  (during CP molecular dynamics)
• WARNING! XC FUNCTIONALS INCONSISTENT
• STOPGM! STACK OF MAIN CALLS (during geometry optimization)
• Warning! Spline region smaller than maximum G-value
  (during geometry optimization with variable cell)
• GORDER| PROGRAMING ERROR. INFORM THE PROGRAMER (during startup of WAVEFUNCTION or GEOMETRY optimization)

Q: Could anybody tell me the following error in the cpmd output during CP Molecular Dynamics runs with the flag WANNIER WFNOUT LIST DENSITY?

WANNIER CODE WARNING: GRADIENT FOR RESTA FUNCTIONAL IS GMAX=0.118E-02

Does it mean any serious error in the calculation?

A: The default spreadfunctional used in CPMD is the Vanderbilt type. At the end of the calculation the convergence with respect to the Resta type functional is also checked. For large cells both should be converged at the same time. However, for typical application this is not the case and you get the warning. This is not serious and you can ignore it.

Q: A warning message appeared in the output http:

 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 ?WARNING! XC FUNCTIONALS INCONSISTENT FOR h.pp
 ?!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Does it mean that my pseudopotential http is wrong? I used fhi98PP to create this pp http, and just recast it. So if it is wrong, what more should I do to the output http of fhi98PP?

A: It means that the XC functional used to generate the pseudo potential and the functional which you want to use in CPMD are not the same; it could be more or less serious: Some of the XC functionals in CPMD are just minor variations of each others, e.g. the LDA part is evaluated using Perdew-Wang'92, Perdew-Zunger'7? or the Pade-interpolation formula. If this is the case, the resulting error is usually small, however if your pseudos were generated e.g. with PBE and you try to use BLYP, the error might become serious. I suggest you to see what are the two functionals (pp & CPMD-input), and look if the difference is significant or not.

Q: When I am running a GEOMETRY OPTIMIZATION using the keywords LSD and MULTIPLICITY, I get the following output:

 ================================================================
 =                  GEOMETRY OPTIMIZATION                       =
 ================================================================
 NFI      GEMAX       CNORM           ETOT        DETOT      TCPU
 EWALD SUM IN REAL SPACE OVER  1* 1* 1 CELLS
 STOPGM! STACK OF MAIN CALLS:
 STOPGM! CALL    FORCEDR
 STOPGM! CALL     FORCES
 STOPGM! CALL    VOFRHOB
 STOPGM! CALL      GCLSD

 PROGRAM STOPS IN SUBROUTINE LSD_GGAX| NOT PROGRAMMED

I would like to know how to solve this problem?

A: This simply means that the LSD version of this specific functional has not been implemented (yet). Feel free to implement it yourself and submit a patch.

Possible solutions are:

1. you implement it into CPMD (see http lsd_func.F)
   
2. you switch to the PBE functional, which is the modern variant of this functional and is implemented both for spin-restricted and unrestricted cases.

Q: I am trying to optimize a crystal structure (both ion positions and cell volume) using CPMD and get the warning message:
Warning! Spline region smaller than maximum G-value.
The optimization seems to converge nicely but what does this warning mean/imply?

A: This in fact touches several points. The following applies also to other variable cell calculations with CPMD (e.g. constant pressure simulations).

• Pseudopotential functions in CPMD are calculated on a radial grid in G-space and then used in spline interpolations. This speeds up variable cell calculations considerably. The maximum grid point is given by the cutoff. With the keyword

     SPLINE RANGE
       x.xx
  

  you can enlarge the grid to x.xx times the cutoff. Also, you should make sure, that the number of spline points is large enough. Older version of CPMD defaulted to as little as 501. This is at the lower limit for accuracy with a fixed cell. Especially if you have high cutoffs it is better to increase this value, e.g.

     SPLINE POINTS
       2500
  

  or larger. The current default (5000) should be large enough.

• In a variable cell calculation, CPMD uses a constant number of plane waves. Therefore if your cell contracts the cutoff increases, if the cell gets larger the cutoff decreases. So if you have the first case the spline interpolation needs points above the original cutoff and you get a warning. Depending on the amount of change in the cell you expect a value for the SPLINE RANGE of 2-5 is needed.

• Coming back to the constant number of plane waves. If your cell gets larger the effective cutoff decreases. This may have very undesirable effects and it is better to define the plane waves on a box larger than any box you anticipate the simulation will reach. In order to not have to start with an unreasonable cell you can define the plane waves not with the actual box but with a reference cell, use the keyword

     REFERENCE CELL
       a b c xa xb xc
  

• However, you really want to do a constant cutoff calculation, not a constant number of plane waves. For technical reasons this is not possible and in principle you should do the calculation at a high enough cutoff in order that the calculation is converged all along the simulation path (with the slight changes in cutoff).

  To avoid these very high cutoffs the group in Trieste came up with a method that allows to perform pseudo constant cutoff calculations. This method is implemented in CPMD (keyword CONSTANT CUTOFF) and explained in the paper [147]

Q: I was optimizing wavefunctions for my system. After a successful run I modified CELL VECTORS. This latter calculation crashed with the following error either when I restarted (only wavefunctions) or if I started from scratch.

 GORDER| PROGRAMING ERROR. INFORM THE PROGRAMER

PROGRAM STOPS IN SUBROUTINE GORDER| ERROR IN G-VEC ORDERING (NHG) [PROC= 0]

Is there something wrong with the code?

A: This is a known problem in CPMD. The message comes from a test, which probes whether the G vectors are in a ``safe'' order, namely such that after a restart with a different number of processes or on a different machine the results agree. Usually this error only occurs in large systems with a high cut-off energy and/or large unit cells, i.e. where one gets lots of close-lying G vectors.

There are two possible workarounds:

1. Slightly change your computational box in one dimension e.g. from $10.000000$ to $10.000001$ . This helps some times.
2. The check is not 100% accurate. This means by just ignoring the message you will most likely get correct results. The error would only appear in restarts where you could see a small inconsistency in energy in the first step. Final results should not be affected (except for MD if you do restarts).

   To avoid the stop, comment out the two lines at the end of http lodapa.F of the form

           CALL STOPGM('GORDER','ERROR IN G-VEC ORDERING (NHG)')
   

   However, be sure to check that the results are reasonable.