[CPMD-list] imaginary frequency for H2O and (H2O)2

[Christian Tuma]

Hello,

On Mon, Jul 11, 2005 at 09:33:22PM -0400, Lan-Feng Yuan (lyuan at Princeton.EDU) wrote:
>   I am trying to get harmonic vibrational frequencies of H2O and (H2O)2 molecules. Most of the calculated values agree with experimental values well, but there is always one imaginary for (H2O)2. For example, use "VIBRATIONAL ANALYSIS LR GAUSS", "CONVERGENCE ORBITALS 1.D-8" in &CPMD, 10 A cubic cell, "SYMMETRY 1", cutoff 100 Ry, "FUNCTIONAL PBE",

Why "SYMMETRY 1" ? For gas phase molecular systems you probably want
"SYMMETRY 0".

> in the output file I get:
> ... 
>
>  PURIFICATION OF DYNAMICAL MATRIX
> 
>  ****************************************************************
>  HARMONIC FREQUENCIES [cm**-1]:
> 
>        -227.8840       -130.5718        -82.8693        -50.1953
>           0.0000          0.0000          0.0000        154.2157
>         185.2528        360.3831        433.5914        639.7611
>        1580.6185       1609.7543       3548.3839       3723.8365
>        3804.1202       3824.0986

Again, you have a gas phase molecular system, therefore you might want
to project out translational _and_ rotational degrees of freedom using
"ISOLATED MOLECULE". This is done per default when switching to "SYMMETRY 0".

>   And in the Gaussian-like VIB1.log it shows the -50.1953 as the only imaginary frequency.

Take a look into the manual (or source code) to see which modes are included
in VIB1.log and in VIB2.log: VIB1.log does not contain the lowest three modes.

> The input atomic positions are optimized using "OPTIMIZE GEOMETRY", "CONVERGENCE ORBITALS 1.D-8", and "CONVERGENCE GEOMETRY 4.D-4". Here a point noteworthy is that I cannot make the geometry convergence threshold any smaller, otherwise it will run forever with basically unchanged total energy and gradients. Maybe this problem has to do with the imaginary frequency ...

Maybe now you can optimize a little better than only 4.D-4. Did you try
other optimisers like "LBFGS"?

> ...
>   Here the 3 imaginary frequencies disappear in VIB1.log. But anyway, is there any problem with purification of dynamical matrix?

Yes, see above.

Good luck,
Christian.

-- 
Christian Tuma             Humboldt-Universitaet zu Berlin
ct at chemie.hu-berlin.de     Arbeitsgruppe Quantenchemie (Prof. Sauer)
phone: +49-30-20937140     Brook-Taylor-Str. 2, 12489 Berlin, GERMANY
fax:   +49-30-20937136     http://www.chemie.hu-berlin.de/ag_sauer