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

[Lan-Feng Yuan (lyuan@Princeton.EDU)]

  Hi, dear CPMD community,

  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", 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
----------------------------