Re: How does the Argon melting temperature depend on density

If I correctly understand the description of what you are doing, I think 
there is a fundamental problem in your simulation constraints.

It does not make sense to simulate melting in a constant desnity ensemble, 
because the densities of the coexisting solid and liquid are different.  
People who do molecular dynamics simulation of melting (I am not such a 
person, but I have done a little simulation work) typically work in a 
constant *pressure* ensemble, since that variable is the same in the 
coexisting phases.

Since it sounds like you are at a university (and therefore should have 
access to literature), I will give a few recent references of people who 
have published MD studies of melting for simple fluids; you can see these 
papers for more hints on how to do it:

J. Solca et al., J. Chem. Phys., 108, 4107 (1998)
H.L. Tepper & W.J. Breits, J. Chem. Phys., 115, 9434 (2001)
S.N. Luo et al., J. Chem. Phys., 120, 11640 (2004)