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

[Alexander Isaev]

 Dear Lan-Feng, 
 These are 3N vibrational frequencies, you should subtract 
 rotational and translational degrees of freedom (first six 
 frequencies). Thus for water, for instance, you will have only 3  frequency: 
 1597.9613       3735.7874        3826.2417 
 
 That is consistent with e.g. Gaussian output. 
 
 All the best, 
 Alexandr Isayev 
 
 
 
 
> ---------- Original Message ---------------------------------- 
> From: "Lan-Feng Yuan (lyuan at Princeton.EDU)" <lyuan at Princeton.EDU> 
 Date:  Mon, 11 Jul 2005 21:33:22 -0400 
 
 >  Hi, dear CPMD community, 
 > 
 >  I am trying to get harmonic vibrational 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", in the output file I get: 
> > 
> > HARMONIC FREQUENCIES [cm**-1]: 
> > 
> >       -198.2237       -151.1669       -117.8813       -117.3705 
> >        -50.7229        120.2540        154.6051        157.5825 
> >        205.6228        313.9255        387.8642        662.1068 
> >       1583.5806       1605.0412       3545.6478       3723.9891 
> >       3802.3660       3828.0855 
> > 
> > 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 
> > 
> >  And in the Gaussian-like VIB1.log it shows the -50.1953 as the 
> only imaginary frequency. 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 ... 
> > 
> >  For a H2O molecule, I get: 
> > 
> > HARMONIC FREQUENCIES [cm**-1]: 
> > 
> >       -317.3444       -203.7113        -97.7685        -26.2296 
> >        -16.1963        127.8522       1584.6342       3733.1835 
> >       3841.6947 
> > 
> > PURIFICATION OF DYNAMICAL MATRIX 
> > 
> > **************************************************************** 
> > HARMONIC FREQUENCIES [cm**-1]: 
> > 
> >       -634.4127       -242.9876        -16.1971          0.0000 
> >          0.0000          0.0001       1597.9613       3735.7874 
> >       3826.2417 
> > 
> >  Here the 3 imaginary frequencies disappear in VIB1.log. But 
> anyway, is there any problem with purification of dynamical matrix? 
> > 
> >  Thank you very much! Best regards, 
> > 
> >  Lan-Feng 
> > 
> >---------------------------- 
> >Dr. Lan-Feng Yuan, 
> >Postdoctoral Associate, 
> >Department of Chemistry, 
> >Princeton University, 
> >Princeton, NJ 08544, USA 
> >Office Phone: 1-609-258-1834 
> >Fax: 1-609-258-6746 
> >---------------------------- 
> > 
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> > 
> 
>