>>>>Hello gmx-users! >>>> >>>>I am still struggling with the energy conservation in the system where a >>>>water droplet (consisting of rigid molecules) collides with a frozen >>>>structured surface. The total energy is well conserved at the beginning of >>>>NVE simulation when the droplet is approaching the surface and once again >>>>when the droplet is stabilized on the surface, but the drift or fluctuation >>>>in the total energy is observed between these extremes. >>>> >>>>The total energy of the system (~300 000kJ/mol) decreases slightly >>>>(<100kJ/mol) at the collision and on non-wetting surfaces (water droplet >>>>wont intrude to the pores of the surface) part of the energy reverts quickly >>>>causing a sharp peak in the energy curve. This could indicate an >>>>integration error at the collision due to the large time step, but equal >>>>behavior is observed with the time steps of 2.0fs and 0.5fs. >>>> >>>>On the wetting surfaces (water droplet intrude to the pores of the surface) >>>>the center of mass motion of the droplet come to a stop at the collision >>>>as in the case of non-wetting surface and sharp decrease in the total energy >>>>is observed. However, now none of this energy is reverted and the total >>>>energy stabilize to this value until the water molecules starts to intrude >>>>to the pores of the surface. The total energy decrease slowly in conjuction >>>>with the slow sinking of the height of the center of mass of the water >>>>droplet until the bottom of the pores is reached. After the bottom of the >>>>pores is reached energy will stabilize to the corresponding value which is >>>>about 200-300kJ/mol lower than atthe beginning. >>>> >>>>Decreasing the time step from 2.0fs to 0.5fs have no influence for the >>>>energy conservation and somehow the problem seems to be connected to >>>>water-surface interactions: on the wetting surfaces (more water-surface >>>>interactions) a drift is observed instead of a sharp peak of non-wetting >>>>surfaces (less water-surface interactions). Any ideas how I could achieve >>>>complete energy conservation? >>>> >>>>Thanks for your time and help, >>>> >>>>Janne >>> >>>What kind of interactions are you using? >>>Plain cut-off for Coulomb and LJ will give bad energy conservation. >>>For your system you probably want to use shifted LJ and Coulomb potentials. >>> >>>Berk. >> >>I am using PME with slab correction for Coulombic interactions and plain >>cut-off for Lennard-Jones interactions. Neighbor lists are updated every time >>step due to the use of plain cut-off for LJ interactions. This actually gives >>really good energy conservation at the beginning of the simulation, when the >>droplet is approaching the surface and once again when the droplet is >>stabilized on the surface, but the problems exist between these "stable >>situations". > >You don't specify how you made your surface, >but I assume it would be with LJ particles. >Then you have jumps in the energy with a cut-off, since the LJ potentials >is not zero at the cut-off. This will result in lower energies at contact. > >Use a shifted potential, in that way you can also set nstlist to something >like 10 which will speed up your simulations. > >Berk.
Thanks Berk! The total energy seems to be conserved at the collision when a shifted or switch function is used for LJ interactions. Comparison simulations without an impact velocity in NVT ensemble used a pure cut-off for LJ interactions but I assume that this small change in the "shape of potential function" won't affect the physics of the system but I still have to check it... Janne PS. I have some problems with my email and hope that this isn't a duplicate message... _______________________________________________ gmx-users mailing list gmx-users@gromacs.org http://www.gromacs.org/mailman/listinfo/gmx-users Please search the archive at http://www.gromacs.org/search before posting! Please don't post (un)subscribe requests to the list. Use the www interface or send it to [EMAIL PROTECTED] Can't post? Read http://www.gromacs.org/mailing_lists/users.php
