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[Masayoshi Mikami <mmikami@rc.m-kagaku.co.jp>]

Hi, "Nee how,"

I do not know how it went bad, but my suggestion is :
System which has peculiar (non-classical) chemical bond
may have such difficulty with SCF convergence.

In your case, C-O bond length is 3.85 x 2 = 7.7 bohr
= 4.07 angstrom, which is much longer than typical
C-O bond length in CO (about 1.2 angstrom ? Kindly check it).
So, initial DEN/WFK may be carefully prepared, e.g.,
starting the optimized structure, then running a series of
SCF calculation by increasing the C-O bond gradually (carefully!)
wherein the initial DEN/WFK for the n-th SCF calculation
can be obtained from the (n-1)-th SCF calculation result.

In passing, its spin state should also be carefully examined...
(Please take a look into the Test_*, some of which may be
 revised to be useful for your case...)

Good luck,
Masayoshi

On 2005/08/03, at 11:11, occupierliu@yahoo.com.cn wrote:

> Dear abinit user:
>     When I try to calculate the total energy of two atoms at ground 
> state, the abinis version 4.4.3 break off undesignedly. The error 
> message in the log file is:
>   vtowfk : BUG   For istwf_k=2, observed the following element of evec 
> :
>     3    1             nan             nan
>   with a non-negligible imaginary part.
>   Action : contact ABINIT group.
>
>  Delivered    6 WARNINGs and   0 COMMENTs to log file.
>
>  leave_new : decision taken to exit ...
>
> with warnings like:
>  scfcge: WARNING -
>   Potential-based CG line minimization has trouble to converge.
>   The algorithm is restarted with more secure parameters.
>
> I've changed the iscf from default 5 to 3, but it didn't work.
> Do I need to change some parameters? I appreciate any advice.
>
>
>
> Input file:
> # C-O potential energy calculation
> #
>
> #Definition of the unit cell
> acell: 40 34 34   # The keyword "acell" refers to the
>                   # lengths of the primitive vectors (in Bohr)
>
> #Definition of the atom types
> ntypat 2          # There is only one type of atom
> znucl 8 6         # The keyword "znucl" refers to the atomic number of 
> the
>                   # possible type(s) of atom. The pseudopotential(s)
>                   # mentioned in the "files" file must correspond
>                   # to the type(s) of atom. Here, the only type is 
> Hydrogen.
> nband 10
> getwfk -1
>
> #Definition of the atoms
> natom 2           # There are two atoms
> typat 1 2         # They both are of type 1, that is, Hydrogen
> xcart
>   -3.85 20.0 20.0
>    3.85 20.0 20.0
> #Definition of the planewave basis set
> ecut 30.0         # Maximal plane-wave kinetic energy cut-off, in 
> Hartree
>
> #Definition of the k-point grid
> nkpt 1            # Only one k point is needed for isolated system,
>                   # taken by default to be 0.0 0.0 0.0
>
> #Definition of the SCF procedure
> #iscf  3
> nstep 1000         # Maximal number of SCF cycles
> toldfe 1.0d-12   # Will stop when, twice in a row, the difference
>                   # between two consecutive evaluations of total energy
>                   # differ by less than toldfe (in Hartree)
> diemac 1.0        # Although this is not mandatory, it is worth to
>                   # precondition the SCF cycle. The model dielectric
> diemix 0.5        # function used as the standard preconditioner
>                   # is described in the "dielng" input variable 
> section.
>                   # Here, we follow the prescriptions for molecules
>                   # in a big box