Because ΔG values in primer design are very important in order to prevent homodimerisation primer dimers etc, I always check my primers before ordering them. I try as much as possible to avoid those with high primer dimers or hairpins. During check sometime I find the ΔG is positive and sometime is negative. Do not what is the acceptable value that I can depend on.

I am looking for anyone has idea about this and it will be great if there is a reference regarding this.