Yes,

Check you data file, and find out how many types of impropers you have.

For each type you should have a corresponding coeff line in the input script.

As for LAMMPS documentation:

https://docs.lammps.org/improper_harmonic.html

improper_coeff 1 100.0 0

The first value is the improper type, the second is the force constant and the third is the equilibrium angle.

Israel Zadok thank you so much sir for your answer. I generated my data file from LigParGen Server (yale.edu) to apply the OPLS force field. The improper coefficient have 4 values unlike the one stated in the documentation. Do you perhaps know what does the fourth value represents?

Amirah Fuad, this looks like CVFF improper style coefficients, look here:

https://docs.lammps.org/improper_cvff.html

You can just change the style by changing:

improper_style harmonic --> improper_style cvff

Israel Zadok thank you so much sir. Much appreciated.