[CPMD-list] transition state of a molecule

[Martin Konopka]

Dear CPMD community,

I am trying to locate a transition state of the CH3-N-N-CH3 molecule
using the P-RFO optimizer. However, I can not see any progress towards
the expected TS. The molecule essentially remains in its starting
structure. In more details, the molecule has two isomers,

  the cis one:           the trans one:

                                    C
                                   /
   N ------ N                     /
    /      \             N ------ N
   /        \             /
  C          C           /
                        C

and a potential energy barrier between them. I am mostly starting from
the slightly randomized cis structure and trying to locate the top
of the barrier. I tried several choices for reaction core and mode to be
followed. Regardless of setting the mode, the result remains the same and
unsatisfactory. In some cases the atoms in the reaction core do not move
at all (for example in a special case when I started from a
hand-modified configuration presumably closer to the TS.)

I would greatly appreciate some hint to this problem.
Below is my input file and modes from vibrational analysis.

I tried the 4th, 7th, 10th mode and also the 1st one, which is the
default. I also have a question: If I use, for example
PRFO MODE
 10
does it really mean that the mode corresponding to the 10th frequency of
vibrational analysis is followed? If yes, then using PRFO MODE less than
7 (or 6 in special cases), which is the default, would have no sense since
it corresponds to overall translations or rotations. Or do I
misunderstand something?


&CPMD
  OPTIMIZE GEOMETRY XYZ
  INITIALIZE WAVEFUNCTIONS ATOMS
  RANDOMIZE COORDINATES
   0.1D0
  RESTFILE
    4
  STORE
    100
  LBFGS
  PRFO MODE
    10
  PRFO CORE=4
    1 2 9 10
  CONVERGENCE
    5.0D-7 5.0D-5
  CONVERGENCE RELAX
    15
  SPLINE POINTS QFUNCTION
    2001
  ISOLATED MOLECULE
  CENTER MOLECULE OFF
  MEMORY BIG
  LSD
&END

&DFT
  FUNCTIONAL PBE
  GC-CUTOFF
    5.0D-5
&END

&SYSTEM
  SYMMETRY
    0
  POISSON SOLVER TUCKERMAN
  ANGSTROM
  CELL
    10.0D0  1.0D0  1.0D0  0.0D0  0.0D0  0.0D0
  CUTOFF
    25.0D0
  MULTIPLICITY
    1
&END

&ATOMS
*N.uspp_ascii FORMATTED
   LMAX=P
    2
     4.525338176055      4.920747785642      4.949904563339
     5.765416638683      5.048851794396      5.079925564279
*H.uspp_ascii FORMATTED
   LMAX=S
    6
     4.265466440716      2.788011128505      4.858413782037
     2.878604693260      3.791408955331      4.306112314885
     4.333398001463      3.598265838244      3.267273093363
     6.467823612522      3.016976904998      5.093103687689
     6.554809448253      3.823511424486      3.501228774397
     7.703325625270      4.291321957329      4.803526469019
*C.uspp_ascii FORMATTED
   LMAX=P
    2
     3.972567677301      3.700817361171      4.307476081948
     6.672717274443      3.979768413937      4.589493362517
&END


vib. ana. for the optimized cis structure, after purification:

 HARMONIC FREQUENCIES [cm**-1]:

          0.0000          0.0000          0.0000          0.0000
          0.0000          0.0000         89.1253        160.4933
        342.9720        461.3605        603.3003        854.8066
        894.5454        918.9702       1033.3395       1041.1772
       1122.1006       1310.9720       1322.4554       1397.1613
       1406.4886       1414.6120       1437.0546       1535.1880
       2896.9570       2900.3452       2961.5598       2972.3951
       3036.4163       3036.7126

(The 10th mode, 461.3605 cm^-1 is particularly interesting since it
changes the dihedral angle of CNNC.)
I also attach my output file for convenience.

Thanks.
Martin Konopka.
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