Re: [ESPResSo-users] Sistematically decreasing of Temperature with DPD thermostat

[Georg Rempfer]

Hello Salvador,

you might be experiencing the flying ice cube phenomenon (http://en.wikipedia.org/wiki/Flying_ice_cube).

I do not know the implementation of the DPD thermostat in Espresso very well though and may be wrong.

Greetings,

Georg

2014/1/14 Salvador H-V <address@hidden>

Dear All,

I am performing simulations of a two dimensional system of hard dummbbells.

I am using a generalized LJ potential to simulate the hard interaction and rigid bonds to constraint the two center of the spheres that composed one dimer.

 I was using LD thermostat but as I am interested in dynamical properties I decided to switch to DPD thermostat. Here it is my [code_info] compilation:

 Compilation status { FFTW } { CONSTRAINTS } { BOND_ANGLE_HARMONIC } { GAUSSIAN } { HERTZIAN } { PARTIAL_PERIODIC } { LENNARD_JONES } { MODES } { EXTERNAL_FORCES } { TABULATED } { SOFT_SPHERE } { LENNARD_JONES_GENERIC } { MASS } { ELECTROSTATICS } { BOND_CONSTRAINT } { DPD } { TRANS_DPD } { EXCLUSIONS } }"

I am running the same program that I previously used with LD only with the following modifications:

thermostat set off

galilei_transform

set dpd_cut [ expr { ( pow ( 2.0 , 1.0/6.0 ) )*$diam } ]

thermostat dpd $temp_ave 0.0 $dpd_cut WF 0 1.0 $dpd_cut TWF 0

(dpd 1.5 0.0 1.122462048309373 WF 1 1.0 1.122462048309373 TWF 0 )

When I switch form LD to DPD (standard or transversal) thermostat, I get a sistematically decrease of the temperature: set temp [expr [analyze energy kinetic]/($np)]

un 100 at time=49.99999999997417 E = 1.464997510334239 & Temp = 1.478570945756926^M

run 200 at time=59.99999999995087 E = 1.445825470054056 & Temp = 1.463115509638189^M

.

.

.

run 124200 at time=12460.00000207969 E = 0.01086010276683752 & Temp = 0.02004077077749828^M

run 124300 at time=12470.00000208172 E = 0.01324369725959244 & Temp = 0.02005819667634172

I don't understand what is going on? Any idea or suggestion would be greatly appreciated.

Thanks a  lot in advance,

Salvador