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

[Axel Kohlmeyer]

On Mon, 11 Jul 2005, Lan-Feng Yuan (lyuan at Princeton.EDU) wrote:

LY>   Hi, dear CPMD community,

dear lan-feng,

some additional comments.

LY>   I am trying to get harmonic vibrational frequencies of H2O and
LY> (H2O)2 molecules. Most of the calculated values agree with
LY> experimental values well, but there is always one imaginary for
LY> (H2O)2. For example, use "VIBRATIONAL ANALYSIS LR GAUSS", "CONVERGENCE
LY> ORBITALS 1.D-8" in &CPMD, 10 A cubic cell, "SYMMETRY 1", cutoff 100
LY> Ry, "FUNCTIONAL PBE", in the output file I get:

you may also consider using SYMMETRY 0 
even though you use a large supercell there are probably 
still weak but significant interactions with the periodic 
images that result in some of those low frequencies.
also you are probably a 'victim' of the 'ripples' effect
(see below).

LY> 
LY>  HARMONIC FREQUENCIES [cm**-1]:
LY> 
LY>        -198.2237       -151.1669       -117.8813       -117.3705
LY>         -50.7229        120.2540        154.6051        157.5825
LY>         205.6228        313.9255        387.8642        662.1068
LY>        1583.5806       1605.0412       3545.6478       3723.9891
LY>        3802.3660       3828.0855
LY> 
LY>  PURIFICATION OF DYNAMICAL MATRIX
LY> 
LY>  ****************************************************************
LY>  HARMONIC FREQUENCIES [cm**-1]:
LY> 
LY>        -227.8840       -130.5718        -82.8693        -50.1953
LY>           0.0000          0.0000          0.0000        154.2157
LY>         185.2528        360.3831        433.5914        639.7611
LY>        1580.6185       1609.7543       3548.3839       3723.8365
LY>        3804.1202       3824.0986
LY> 

LY>   And in the Gaussian-like VIB1.log it shows the -50.1953 as the only
LY> imaginary frequency. The input atomic positions are optimized using
LY> "OPTIMIZE GEOMETRY", "CONVERGENCE ORBITALS 1.D-8", and "CONVERGENCE
LY> GEOMETRY 4.D-4". Here a point noteworthy is that I cannot make the
LY> geometry convergence threshold any smaller, otherwise it will run
LY> forever with basically unchanged total energy and gradients. Maybe
LY> this problem has to do with the imaginary frequency ...

the accuracy of the geometry optimization is also affected by
the various grids that are used for the wavefunction and the
electronic density. although cpmd uses spline interpolation to
not have any discontinuities, you still have a so-called 'ripple'
effect, i.e. indirect forces due to the use of a grid at all.
you may be able to lower that by increasing number of grid points
(indirectly) by increasing the plane wave cutoff. also, for an
isolated system calculation with a gradient corrected functional,
you tend to pick up some (unphysical) contributions to the gradient
correction from noise in the vacuum region. this should be reduced
by increasing the cutoff as well, but can also be dampened by 
(moderately) increasing the GC-CUTOFF parameter. 

best regards,
	axel.

LY> 
LY>   For a H2O molecule, I get:
LY> 
LY>  HARMONIC FREQUENCIES [cm**-1]:
LY> 
LY>        -317.3444       -203.7113        -97.7685        -26.2296
LY>         -16.1963        127.8522       1584.6342       3733.1835
LY>        3841.6947
LY> 
LY>  PURIFICATION OF DYNAMICAL MATRIX
LY> 
LY>  ****************************************************************
LY>  HARMONIC FREQUENCIES [cm**-1]:
LY> 
LY>        -634.4127       -242.9876        -16.1971          0.0000
LY>           0.0000          0.0001       1597.9613       3735.7874
LY>        3826.2417
LY> 

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

LY> 
LY>   Thank you very much! Best regards,
LY> 
LY>   Lan-Feng
LY> 
LY> ----------------------------
LY> Dr. Lan-Feng Yuan,
LY> Postdoctoral Associate,
LY> Department of Chemistry,
LY> Princeton University,
LY> Princeton, NJ 08544, USA
LY> Office Phone: 1-609-258-1834
LY> Fax: 1-609-258-6746
LY> ----------------------------
LY> 
LY> _______________________________________________
LY> CPMD-list mailing list
LY> CPMD-list at cpmd.org
LY> http://cpmd.org/mailman/listinfo/cpmd-list
LY> 
LY> 

-- 

=======================================================================
Dr. 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.