Hi Steve / Ahmed

I think I found what the problem is. I realise that if I used the "hybrid" in pair style, i.e.

pair_style hybrid lj/cut/coul/long/tip4p 1 2 1 1 0.125 9 8

pair_coeff 1 1 lj/cut/coul/long/tip4p 0.16275 3.16435

pair_coeff 1 2 lj/cut/coul/long/tip4p 0.0 0.0

pair_coeff 2 2 lj/cut/coul/long/tip4p 0.0 0.0

then with NPT, I would not get a converged result. However, if I remove "hybrid" in pair style, i.e.

pair_style lj/cut/coul/long/tip4p 1 2 1 1 0.125 9 8

pair_coeff 1 1 0.16275 3.16435

pair_coeff 1 2 0.0 0.0

pair_coeff 2 2 0.0 0.0

then the results would converge, and the density of water I obtain would be correct.

My question is, if I were to perform a hybrid calculation with two pair styles, would the computation from LAMMPS be correct?

Thanks

Huck Beng