about md in npt

From: Hasan haska (hasanhaska_at_yahoo.com)
Date: Wed Dec 28 2011 - 02:18:30 CST

  Dear Namd users, I run a 5 ns md simulation in NPT ensamble for 1114 glycol molecules to equilibrate the molecules. After the md simulation I looked over the pressure with namdplot and saw that pressure fluctuates between 310743.7 and -1800.41 atm and the averaged fluctuated pressure value is 720 atm which is too high although the target pressure is 1 atm. What is the reason for this ? Could you please make an explanation about this issue ? Thank you very much for your attention.  Best regards.   #--- This is a test namd configuration file paratypeCharmm on parameters gyloPRM.prm structure ETG.psf coordinates ETG_.pdb outputName ETGaa1_1out set temperature 300 temperature $temperature   firsttimestep 0 # Integrator Parameters timestep 1 nonbondedFreq 1 fullElectFrequency 5 stepspercycle 10       # Force-Field Parameters exclude scaled1-4 1-4scaling 0.8333333 cutoff 19.7 switching on switchdist 17 pairlistdist 24   # Constant Temperature Control nvt langevin on ;# do langevin dynamics langevinDamping 5 ;# damping coefficient (gamma) of 5/ps langevinTemp $temperature langevinHydrogen on ;# don't couple langevin bath to hydrogens   #--- PBC   cellBasisVector1 40. 0.0 0.0 cellBasisVector2 0.0 40. 0.0 cellBasisVector3 0.0 0.0 65. #--- PME dielectric 1 PME on PMEGridSizeX 45 PMEGridSizeY 45 PMEGridSizeZ 72   # Constant presssure Control (variable volume) useGrouppresssure no ;# needed for 2fs steps useFlexibleCell yes useConstantArea yes ;# langevinPiston on langevinPistonTarget 1.01325 ;# in bar -> 1 atm langevinPistonPeriod 200. langevinPistonDecay 100. langevinPistonTemp $temperature   #--- Output & ETGtart binaryoutput no binaryrestart yes restartname ETG_1outrestart DCDfile ETG_1out.dcd   ETGtartfreq 1250 dcdfreq 1250 xstFreq 1250 outputEnergies 1250 forceDCDfreq 1250 outputpresssure 1250 outputtiming 1250   minimize 12500 reinitvels $temperature   run 5000000

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