[Pw_forum] Temperature and pressure control in CP dymics of example 18

[Lily Anh]

Dear all,

I would like to study MD with NPT ensemble. I am learning example 18 in QE 4.2.1 which uses cp.x to perform molecular dynamics simulation of SiO2.

For the vc-cp simulation, input file specify tempw=300 and press=1.0. However, the   Averaged Physical Quantities
                      accomulated      this run
   ekinc         :        0.00001       0.00001 (AU)
   ekin          :       94.60238      94.60238 (AU)
   epot          :     -343.05127    -343.05127 (AU)
   total energy  :     -211.98234    -211.98234 (AU)
   temperature   :        0.75835       0.75835 (K )
   enthalpy      :     -211.93045    -211.93045 (AU)
   econs         :     -211.93020    -211.93020 (AU)
   pressure      :       -2.19645      -2.19645 (Gpa)
   volume        :     1526.61474    1526.61474 (AU)

Why the averaged temperature and pressure in the output file are far  away from the target value set in the input? 
One more question, what is the purpose of using cp, cp-restart, vc-cp, and vc-cp-restart together? Is this a routine to study MD, i.e. combing these four? In other words, could we just get the vc-cp part for investigating NPT ensemble?

Best regards
Jingyun





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