def van_vleck_weisskopf(νeval, ν, γ, δ):
    """Complex Van Vleck-Weisskopf line shape function.

    νeval   GHz     frequency at which line shape is evaluated
    ν       GHz     center frequency of line
    γ       GHz     width parameter of line
    δ       -       overlap parameter of line

    Returned value is unitless.
    """
    term1 = (1. - 1j*δ)/(ν - νeval - 1j*γ)
    term2 = (1. + 1j*δ)/(ν + νeval + 1j*γ)
    return νeval * (term1 - term2)