Dear Steve, Axel and lammps users
i would like to use hydrogen bonding potential of DREIDING force field in my MD simulation study of hydrated polymer system. for bettre simulation, first i used this potential in simple system consisted of only 200 water molecules without any polymer chains. after running for 100 ps in NPT (300 K, 1 atm), the final density is about 1.15 g/cm3. when this simple system were run without hydrogen bonding, the density was 1 g/cm3. the force field used for water is flexible three-centered model (F3C). i do not know why this difference in density is occurred when hydrogen bonding is included. the pair_style and pair_coeff commands used for simulation were:
pair_style hybrid/overlay lj/cut/coul/long 10 hbond/dreiding/lj 4 4.5 5 135
pair_coeff 1 1 lj/cut/coul/long 0.01 0.8018 10
pair_coeff 1 2 lj/cut/coul/long 0.043 1.59 10
pair_coeff 2 2 lj/cut/coul/long 0.1848 3.1655 10
pair_coeff 2 2 hbond/dreiding/lj 1 i 4 2.75
the atom types 1 and 2 are hydrogen and oxygen atoms of water, respectively.
i read the lammps doc page for hb/dreiding/lj and dreiding force field paper many times and checked the parametres used for hydrogen bonding. the hydrogen bond energy was about -1300 kcal/mol. i think that the distance (inner and outer) and angle cutoff parameters in hbond pair style are probably the main cauese of higher water density. i do not know how these parameters affect Hbond energy and how to determine them beause they were not reported in DREIDING force field.
please help me
best regards
you are not really asking a LAMMPS question,
but a force field question. You're better off going
to the literature for that.
Steve
You get a difference in density because you are adding an interaction. Besides, water models are constructed to already represent hydrogen bonding; why would you want to add it explicitly to what is already there implicitly? It doesn't make any sense.
Force fields are all about balance in the interactions. This is why you cannot just mix and match them and xpect good results.
Axel