van der Waals damping parameter describes broadening of spectral lines due collisions with neutral particles, e.g. Hydrogen atoms. In VALD3 it can have three values:

if 0: it is not known;

if <0: log γ₆ in units of 1/(s*NH) at l0000 K (where NH is the number density of neutral Hydrogen atoms);

if >0 : two parameters as defined in Barklem et al. 2000A&AS..142..467. Integer part is cross-section σ, fractional part is velocity parameter α.