[CPMD-list] unknown error message

[Axel Kohlmeyer]

>>> "AA" == Abraham Alaka <chokosabe at yahoo.co.uk> writes:

AA> Your CUTOFF value might be too small (~70 might be
AA> better) and your CONVERGENCE ORBITALS a bit too large
AA> (1.0D-7 might do).

in fact, for an oxide and your choice of pseudopotentials
you may need an even higher cutoff (~120ry) for reasonably
converged forces.

well, lowering CONVERGENCE ORBITALS will not help,
since it does not converge at all.

AA> The problem you are referring to might be fixed in
AA> v3.9. Unless you are doing a band structure
AA> calculation, you may find it easier not to use kpoints
AA> and just do a gamma point calculation instead (this is
AA> default and you may need to include more atoms in your
AA> simulation cell).

i agree. you should do some tests with gamma point 
only first (e.g. plane wave cutoff).


AA> Another thing is that you are using an nlcc
AA> pseudopotential; this is normally most useful when
AA> doing spin polarised calculations (i.e you include LSD
AA> in &CPMD and MULTIPLICITY in &SYSTEM).

to the best of my knowledge the logic is the other way
around. for many spin polarized calculations you need
pseudopotentials with NLCC. the reason being, that without
NLCC you don't have much electron density close to the 
atom core, especially if you don't use a semi-core pseudopotential.


AA> I hope you find this useful.

AA> A.

AA>  --- Jin Zhao <zhaojin4576 at gmail.com> wrote: 
>> Dear Axel,
>> 
>> I have tried to use LANCZOS to optimize the
>> wavefunction. And
>> increase the TESR to 2 and 4. The input file is in
>> the attachment. But
>> still doesn't converge after 300 circles. During the
>> process the DETOT
>> almost decrease to mangitude 1e-5, but after that it
>> increase to 1e-0
>> again. Can you give more suggestions? Thank you very

i have a few more suggestions.  see below.

[...]


>> &CPMD
>> OPTIMIZE WAVEFUNCTION
>> CONVERGENCE ORBITALS
>> 1.0e-5
>> LANCZOS DIAGONALISATION  

for the gamma point / cutoff tests you could try:

  ODIIS NO_RESET=30
   5
  LANCZOS DIAGONALIZATION RESET=2
  CONVERGENCE ORBITALS
    1.0e-6
  PRINT FORCES ON


>> &END
>> 
>> &DFT
>> FUNCTIONAL BLYP

adding

GC-CUTOFF
   1.0d-6

here should improve convergence. the BLYP gradient correction
diverges for low electron density, so it is better not 
calculate it there. adjusting this parameter is always a
tradeoff between stability and accuracy of the calculation.
so as a rule of the thumb it should be as low as possible
but high enough to avoid convergence problems. it also
depends a bit on the plane wave cutoff.


>> &END
>> 
>> &SYSTEM
>> ANGSTROM
>> CELL VECTORS
>> 5.916 0.0000  0.0000
>> 0.000 6.4970  0.0000
>> 0.000 0.0000  18.7144

better use:
SYMMETRY
  8
CELL ABSOLUTE
5.916 6.4970 18.7144 0.0 0.0 0.0

this allows CPMD to take advantage 
of the cell symmetry.

>> 
>> CUTOFF
>> 40.0
>> KPOINTS MONKHORST-PACK
>> 2  2  1

with the above change in the cell symmetry,
this setting will result in a single k-point,
so not much improvement over gamma point.
anyway with your cell dimensions, a
3 by 3 by 1 mesh would be more reasonable
resulting in 4 symmetry adapted k-points.

>> TESR
>> 4
>> &END
>> 
>> &ATOMS
>> 
>> *Ti_MT_BLYP_NLCC.psp KLEINMAN-BYLANDER nonlinear
>> core correction 
>> LMAX=D

hmmm, most d-metals have ghost states unless
you use LOCAL=S here. i don't know whether titanium
is an exception. please check the available literature.
there should be some plane wave pseudopotential DFT 
calculations of ti compounds published...


regards,
        axel.

[...]


	
	
		
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--

=======================================================================
Axel Kohlmeyer       e-mail: axel.kohlmeyer at theochem.ruhr-uni-bochum.de
Lehrstuhl fuer Theoretische Chemie          Phone: ++49 (0)234/32-26673
Ruhr-Universitaet Bochum - NC 03/53         Fax:   ++49 (0)234/32-14045
D-44780 Bochum  http://www.theochem.ruhr-uni-bochum.de/~axel.kohlmeyer/
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.