How can I choose the power-law degree exponent?

[GH15zzh]

To generate a power law graph, SNAP provide a parameter p, standing for the power-law degree exponent.
So how can I choose this? Use different a,b,c and d ? If like this, what formula between a,b,c,d and power-law degree exponent?

[SakuraiYuuta]

I guess it's ./PaRMAT -nVertices 3000000 -nEdges 3000000 -output myout.mtx -b 0.0005 -a 0.4999 -c 0.0005 -noDuplicateEdges -memUsage 0.9