All pair coeffs are not set (src/pair_hybrid.cpp:716)


I am working on conducting simulations that involve Carbon Dioxide and alkanes. For alkanes, I use the OPLS force field with the pair_style lj/cut/coul/long, while for Carbon Dioxide, I apply the pair_style mie/cut. Thus, the pair_style description in my input script is:

units real
atom_style full
pair_style hybrid lj/cut/coul/long {cutoff} mie/cut {cutoff}
bond_style harmonic
angle_style harmonic
dihedral_style opls
pair_modify mix arithmetic tail yes

kspace_style ewald ${ksp}

read_data CO2_SAFTMie_data.lmp

pair_coeff 1 1 lj/cut/coul/long 0.066000 3.500000
pair_coeff 2 2 lj/cut/coul/long 0.066000 3.500000
pair_coeff 3 3 lj/cut/coul/long 0.030000 2.500000
pair_coeff 4 4 mie/cut 0.718751867446 3.7410 23.0 6.660

velocity all create {T} {seed} mom yes rot yes dist gaussian
minimize {etol} {ftol} {maxiter} {maxeval}

timestep {dt} fix 1 all npt temp {T} {T} {Tdamp} iso {P} {P} ${Pdamp}
fix 2 all ave/time 1 100 1000 v_pe v_ke v_etotal v_temp v_pres v_vol ave running file thermo1.profile

run ${run_eq}

I appreciate in advance your collaboration to help me understand where my error is.

Where should the mixed parameters between mie/cut and the other atom types come from? LAMMPS does not know how?

Why not use the same pair style for all interactions?

Thanks, for your answer.

I am evaluating combinations of different force fields. I found that the SAFTMie force field provides good predictions for some properties of Carbon Dioxide, and I want to check if the same holds true for mixtures of hydrocarbons and CO2. Indeed, you are correct that Lammps does not know where the other parameters would come from. The only solution I have found so far is to determine them and introduce them into the code using mixing rules. However, I still have the doubt if there is a more precise way to do this

Worse, mixing different force fields like this is generally frowned upon. The preferred choice is to have a single force field to describe all compounds. This way all parameters are derived in a consistent fashion.