[CPMD-list] vibrational analysis problem

[Davide.Provasi at mi.infn.it]

 Dear cpmd users,

 we are studying a fcc metal with 8 sodium atoms in the elementary cell.

 Starting from a configuration with very small ionic gradients,
 the frozen-phonon vibrational analysis gives positive frequencies;
 but while investigating an instability signalled by another dft code
 (pwscf) we found that moving the atoms along one direction, the total
 energy decreases!

 Do you have any suggestion about why this could happen?
 We use the 3.7.0 version, the CPMD and the SYSTEM sections are:

 &CPMD
 ...
 LANCZOS DIAGONALISATION
 LANCZOS PARAMETER
 80 8 0 1.0D-8
 &END

 &SYSTEM
 STATES
 8
 SYMMETRY
 2
 CELL
 20.66  0.0  0.0   0.0 0.0 0.0
 CUTOFF
 30.
 TESR
 8
 KPOINTS MONKHORST-PACK
 6 6 6
 &END

 the vibrational analysis gives:

HARMONIC FREQUENCIES [cm**-1]:

        111.0800        114.4757        117.3849        117.6129
        124.1423        126.5976        126.9388        138.9402
        139.2090        145.9753        146.5589        150.6921
        173.6433        183.7564        189.9268        190.5546
        201.1518        201.1939        211.6997        217.7247
        230.0807        230.2023        265.7553        270.3765

 PURIFICATION OF DYNAMICAL MATRIX

 ****************************************************************
 HARMONIC FREQUENCIES [cm**-1]:

          0.0000          0.0000          0.0000        111.0807
        114.4758        118.0039        118.2398        125.1590
        127.9322        128.3548        146.5566        150.6920
        173.6432        183.7506        189.4964        190.1283
        201.1518        201.1938        211.6997        217.7247
        230.0797        230.2013        265.7553        270.3713


but moving the coordinates of 0.05 A.U. along the phonon with frequency
114.476 cm^-1, we obtain the following energies:

                                ground state   displaced
(B+E2+X-V)     TOTAL ENERGY =   -1.89976113  -1.89978242   <----------

(B)             BAND ENERGY =   -1.34175466  -1.34169936
(E2=I-H-S+R)  ELECT. ENERGY =   -0.84487243  -0.84491995
(S)                   ESELF =    2.65961520   2.65961520
(R)                     ESR =    0.00000413   0.00000412
(X)     XCORRELATION ENERGY =   -1.37071847  -1.37080495
(V)     XCORRELATION POTEN. =   -1.65758443  -1.65764184


thank you in advance


 ------------------------------------------
|Davide Provasi                            |
|Dept. of Physics, University of Milano    |
|Tel. +39 02 5031 7 717                    |
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