[CPMD-list] "NaN" in path integral molecular dynamics?

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

  Hi, dear community,

  I am practising path integral calculations in CPMD. To my surprise, sometimes the dynamics diverge in several steps generating "NaN" (not a number). For example:

    NFI  EKINC/P  TEMP   EKINP(PRI)  EKINP(VIR)      EKS/P      EQUANT    ECLASSIC        EHAM     TCPU
      1 0.00000    0.8     0.00033     0.04348   -17.17017   -17.16984   -16.79743   -16.79743    15.06
      2 0.00000    3.3     0.00040     0.04295   -17.17038   -17.16998   -16.79743   -16.79743    15.02
      3 0.00000    7.5     0.00051     0.04225   -17.17072   -17.17021   -16.79743   -16.79743    14.99
      4 0.00001   13.3     0.00067     0.04154   -17.17120   -17.17053   -16.79744   -16.79743    14.93
      5 0.00002   20.9     0.00087     0.04055   -17.17180   -17.17093   -16.79744   -16.79742    14.96
      6 0.00009   30.1     0.00112     0.04018   -17.17242   -17.17130   -16.79734   -16.79726    14.98
      7 0.00092   40.9     0.00140     0.03735   -17.17169   -17.17029   -16.79583   -16.79491    14.92
      8 0.01155   53.7     0.00173     0.04254   -17.15014   -17.14841   -16.77265   -16.76110    14.92
      9 0.06071   66.2     0.00210     0.02090   -16.86580   -16.86370   -16.11061   -16.04989    14.98
     10 NaN       86.1     0.00249     0.08526   -14.48683   -14.48433 NaN         NaN            15.00
     11 NaN     NaN        0.00295 NaN         NaN         NaN         NaN         NaN            11.98

  I also note that "EHAM", which is supposed to be conserved to machine accuracy, changed before "NaN" appears. This calculation is for a H2O molecule in a 10 A cubic cell (SYMMETRY 1), cutoff 100 Ry, Trotter Dimension 8, FUNCTIONAL PBE (NEWCODE), and in &CPMD section:

MOLECULAR DYNAMICS
PATH INTEGRAL
NOSE IONS MASSIVE
   300.0  10000.0
NOSE PARAMETERS
   10  3  3  6.0  7  1
NOSE ELECTRONS
   0.001   20000.0

  The initialization of the 8 beads is done by "DEBROGLIE CENTROID 300.0" in &PIMD and "ISOLATED MOLECULE",
"OPTIMIZE WAVEFUNCTION", "PATH INTEGRAL" in &CPMD. What could be the reason? Thank you very much! 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
----------------------------