def calculate_P_virial_np(r_ij):
    """
    The virial between two particles.

    Computes the pairwise Lennard Jones interaction energy based on the separation distance in reduced units.

    Parameters
    ----------
    r_ij : np.array
        The distance between the particles in reduced units.
    
    Returns
    -------
    p_virial : float
        The pairwise Lennard Jones interaction energy in reduced units.

    """
    r6_term = (1./r_ij)** 6
    r12_term = (1./r_ij)** 12
    
    p_virial = -48 * (r12_term - r6_term)
    
    return p_virial