[CPMD-list] about vibrational analysis

[Yong Zhang]

Hi CPMDers,

I have a question about vibrational analysis in cpmd.
My system is one N-methylacetamide molecule (with N-H deuterated to N-D) in
vacuum. The situation is like this: I generated a trajectory (10ps)
following general procedure (with Nose Hoover thermostat at 300K); then 80
snapshots were taken from this traj (one every 100fs for the 8ps); for each
of these snapshots, a wavefunction ompimization was carried out, and based
on this optimized wavefunction (by "restart wavefunction..."), the freq was
calculated using FD method; Then I checked the C=O stretching frequency (so
called amide I mode). Surprisingly, the freq has a distribution over the
range as wide as 400cm-1 (1500cm-1 ~ 1900cm-1). The experimental observation
of this mode in gas phase is 1717cm-1. I did the similar calculation using
charmm force field, the freq distribution is much narrower (1685+/-15 cm-1),
which is more reasonable under harmonic approximation. So my question is,
why the freq dirtribution in CPMD calaulation is so wide?

If quench the snapshots to a minimized structure and do the freq ana, their
freq are close to each other, which might indicate a "single" local minimum.
I also tried LR for freq calculation, the resultant freq is about 10cm-1
lower than the corresponding FD one for all cases; I also tried BO dynamics,
similar results.

Any comments or suggestions are appreciated.

Best wishes
Yong


The input file for dynamics run:
==================
&CPMD
  MOLECULAR DYNAMICS CP
  RESTART WAVEFUNCTION COORDINATES VELOCITIES ACCUMULATORS LATEST

  TEMPERATURE
  300.0
  NOSE IONS MASSIVE
  300.0 1700.0
  NOSE ELECTRONS
  0.002 10000.0

  TIMESTEP
  4.0

  MAXSTEP
  10000

  EMASS
  500.0

  TRAJECTORY XYZ SAMPLE
  5

  DIPOLE DYNAMICS SAMPLE WANNIER
  10
  WANNIER REFERENCE
  0.0 0.0 0.0

&END

&SYSTEM
  ANGSTROM
  SYMMETRY
  1
  CELL
  12.14  1.0  1.0  0  0  0
  CUTOFF
  70.0
&END

&DFT
  NEWCODE
  FUNCTIONAL PBE
  GC-CUTOFF
  5.0e-6
&END

&ATOMS
  ISOTOPE
   1.0
   2.0
   12.0
   14.0
   16.0

*H_MT_PBE.psp   KLEINMAN-BYLANDER
   LMAX=S
   6
   1.69080    .05583    .88430
   3.28199   -.14332    .06344
   2.16794  -1.45255    .00207
  -1.58359    .17630  -2.35128
   -.19701    .20889  -3.48962
   -.49100   1.60271  -2.37391

*H_MT_PBE.psp   KLEINMAN-BYLANDER
   LMAX=S
   1
   -.22859   -.88824   -.93883

*C_MT_PBE.psp   KLEINMAN-BYLANDER
   LMAX=P
   3
   2.18368   -.34882    .01350
   1.56123    .24388  -1.15711
   -.53752    .48601  -2.44731

*N_MT_PBE.psp   KLEINMAN-BYLANDER
   LMAX=P
   1
    .32592   -.14815  -1.44467


*O_MT_PBE.psp   KLEINMAN-BYLANDER
   LMAX=P
   1
   2.16429   1.09559  -1.77796

&END

======================



input file for vib ana:
=====================
&CPMD
  VIBRATIONAL ANALYSIS FD GAUSS SAMPLE
  2
  RESTART WAVEFUNCTION COORDINATES LATEST
&END

&SYSTEM
  ANGSTROM
  SYMMETRY
  1
  CELL
  12.14  1.0  1.0  0  0  0
  CUTOFF
  70.0
&END

&DFT
  NEWCODE
  FUNCTIONAL PBE
  GC-CUTOFF
  5.0e-6
&END

&ATOMS

  ISOTOPE
   1.0
   2.0
   12.0
   14.0
   16.0

*H_MT_PBE.psp   KLEINMAN-BYLANDER
   LMAX=S
   6
   1.69080    .05583    .88430
   3.28199   -.14332    .06344
   2.16794  -1.45255    .00207
  -1.58359    .17630  -2.35128
   -.19701    .20889  -3.48962
   -.49100   1.60271  -2.37391

*H_MT_PBE.psp   KLEINMAN-BYLANDER
   LMAX=S
   1
   -.22859   -.88824   -.93883

*C_MT_PBE.psp   KLEINMAN-BYLANDER
   LMAX=P
   3
   2.18368   -.34882    .01350
   1.56123    .24388  -1.15711
   -.53752    .48601  -2.44731

*N_MT_PBE.psp   KLEINMAN-BYLANDER
   LMAX=P
   1
    .32592   -.14815  -1.44467


*O_MT_PBE.psp   KLEINMAN-BYLANDER
   LMAX=P
   1
   2.16429   1.09559  -1.77796

&END
======================
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